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Manual Reference Pages  -  PERLGLOSSARY (1)

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perlglossary - Perl Glossary



version 5.021009


A glossary of terms (technical and otherwise) used in the Perl documentation, derived from the Glossary of Programming Perl, Fourth Edition. Words or phrases in bold are defined elsewhere in this glossary.

Other useful sources include the Unicode Glossary <>, the Free On-Line Dictionary of Computing <>, the Jargon File <>, and Wikipedia <>.


accessor methods A <B>methodB> used to indirectly inspect or update an <B>objectB>Xs state (its <B>instance variablesB>).
actual arguments The <B>scalar valuesB> that you supply to a <B>functionB> or <B>subroutineB> when you call it. For instance, when you call power("puff"), the string "puff" is the actual argument. See also <B>argumentB> and <B>formal argumentsB>.
address operator Some languages work directly with the memory addresses of values, but this can be like playing with fire. Perl provides a set of asbestos gloves for handling all memory management. The closest to an address operator in Perl is the backslash operator, but it gives you a <B>hard referenceB>, which is much safer than a memory address.
algorithm A well-defined sequence of steps, explained clearly enough that even a computer could do them.
alias A nickname for something, which behaves in all ways as though youXd used the original name instead of the nickname. Temporary aliases are implicitly created in the loop variable for foreach loops, in the $_ variable for map or grep operators, in $a and $b during sortXs comparison function, and in each element of @_ for the <B>actual argumentsB> of a subroutine call. Permanent aliases are explicitly created in <B>packagesB> by <B>importingB> symbols or by assignment to <B>typeglobsB>. Lexically scoped aliases for package variables are explicitly created by the our declaration.
alphabetic The sort of characters we put into words. In Unicode, this is all letters including all ideographs and certain diacritics, letter numbers like Roman numerals, and various combining marks.
alternatives A list of possible choices from which you may select only one, as in, XWould you like door A, B, or C?X Alternatives in regular expressions are separated with a single vertical bar: |. Alternatives in normal Perl expressions are separated with a double vertical bar: ||. Logical alternatives in <B>BooleanB> expressions are separated with either || or or.
anonymous Used to describe a <B>referentB> that is not directly accessible through a named <B>variableB>. Such a referent must be indirectly accessible through at least one <B>hard referenceB>. When the last hard reference goes away, the anonymous referent is destroyed without pity.
application A bigger, fancier sort of <B>programB> with a fancier name so people donXt realize they are using a program.
architecture The kind of computer youXre working on, where one Xkind of computerX means all those computers sharing a compatible machine language. Since Perl programs are (typically) simple text files, not executable images, a Perl program is much less sensitive to the architecture itXs running on than programs in other languages, such as C, that are <B>compiledB> into machine code. See also <B>platformB> and <B>operating systemB>.
argument A piece of data supplied to a <B>programB>, <B>subroutineB>, <B>functionB>, or <B>methodB> to tell it what itXs supposed to do. Also called a XparameterX.
ARGV The name of the array containing the <B>argumentB> <B>vectorB> from the command line. If you use the empty <> operator, ARGV is the name of both the <B>filehandleB> used to traverse the arguments and the <B>scalarB> containing the name of the current input file.
arithmetical operator A <B>symbolB> such as + or / that tells Perl to do the arithmetic you were supposed to learn in grade school.
array An ordered sequence of <B>valuesB>, stored such that you can easily access any of the values using an integer subscript that specifies the valueXs <B>offsetB> in the sequence.
array context An archaic expression for what is more correctly referred to as <B>list contextB>.
Artistic License The open source license that Larry Wall created for Perl, maximizing PerlXs usefulness, availability, and modifiability. The current version is 2. (<>).
ASCII The American Standard Code for Information Interchange (a 7-bit character set adequate only for poorly representing English text). Often used loosely to describe the lowest 128 values of the various ISO-8859-X character sets, a bunch of mutually incompatible 8-bit codes best described as half ASCII. See also <B>UnicodeB>.
assertion A component of a <B>regular expressionB> that must be true for the pattern to match but does not necessarily match any characters itself. Often used specifically to mean a <B>zero-widthB> assertion.
assignment An <B>operatorB> whose assigned mission in life is to change the value of a <B>variableB>.
assignment operator Either a regular <B>assignmentB> or a compound <B>operatorB> composed of an ordinary assignment and some other operator, that changes the value of a variable in place; that is, relative to its old value. For example, $a += 2 adds 2 to $a.
associative array See <B>hashB>. Please. The term associative array is the old Perl 4 term for a <B>hashB>. Some languages call it a dictionary.
associativity Determines whether you do the left <B>operatorB> first or the right <B>operatorB> first when you have XA <B>operatorB> B <B>operatorB> CX, and the two operators are of the same precedence. Operators like + are left associative, while operators like ** are right associative. See Camel chapter 3, XUnary and Binary OperatorsX for a list of operators and their associativity.
asynchronous Said of events or activities whose relative temporal ordering is indeterminate because too many things are going on at once. Hence, an asynchronous event is one you didnXt know when to expect.
atom A <B>regular expressionB> component potentially matching a <B>substringB> containing one or more characters and treated as an indivisible syntactic unit by any following <B>quantifierB>. (Contrast with an <B>assertionB> that matches something of <B>zero widthB> and may not be quantified.)
atomic operation When Democritus gave the word XatomX to the indivisible bits of matter, he meant literally something that could not be cut: X- (not) + -XXXXX (cuttable). An atomic operation is an action that canXt be interrupted, not one forbidden in a nuclear-free zone.
attribute A new feature that allows the declaration of <B>variablesB> and <B>subroutinesB> with modifiers, as in sub foo : locked method. Also another name for an <B>instance variableB> of an <B>objectB>.
autogeneration A feature of <B>operator overloadingB> of <B>objectsB>, whereby the behavior of certain <B>operatorsB> can be reasonably deduced using more fundamental operators. This assumes that the overloaded operators will often have the same relationships as the regular operators. See Camel chapter 13, XOverloadingX.
autoincrement To add one to something automatically, hence the name of the ++ operator. To instead subtract one from something automatically is known as an XautodecrementX.
autoload To load on demand. (Also called XlazyX loading.) Specifically, to call an AUTOLOAD subroutine on behalf of an undefined subroutine.
autosplit To split a string automatically, as the Xa <B>switchB> does when running under Xp or Xn in order to emulate <B>awkB>. (See also the AutoSplit module, which has nothing to do with the Xa switch but a lot to do with autoloading.)
autovivification A Graeco-Roman word meaning Xto bring oneself to lifeX. In Perl, storage locations (<B>lvaluesB>) spontaneously generate themselves as needed, including the creation of any <B>hard referenceB> values to point to the next level of storage. The assignment $a[5][5][5][5][5] = "quintet" potentially creates five scalar storage locations, plus four references (in the first four scalar locations) pointing to four new anonymous arrays (to hold the last four scalar locations). But the point of autovivification is that you donXt have to worry about it.
AV Short for Xarray valueX, which refers to one of PerlXs internal data types that holds an <B>arrayB>. The AV type is a subclass of <B>SVB>.
awk Descriptive editing termXshort for XawkwardX. Also coincidentally refers to a venerable text-processing language from which Perl derived some of its high-level ideas.


backreference A substring <B>capturedB> by a subpattern within unadorned parentheses in a <B>regexB>. Backslashed decimal numbers (\1, \2, etc.) later in the same pattern refer back to the corresponding subpattern in the current match. Outside the pattern, the numbered variables ($1, $2, etc.) continue to refer to these same values, as long as the pattern was the last successful match of the current <B>dynamic scopeB>.
backtracking The practice of saying, XIf I had to do it all over, IXd do it differently,X and then actually going back and doing it all over differently. Mathematically speaking, itXs returning from an unsuccessful recursion on a tree of possibilities. Perl backtracks when it attempts to match patterns with a <B>regular expressionB>, and its earlier attempts donXt pan out. See the section XThe Little Engine That /Couldn(nXt)X in Camel chapter 5, XPattern MatchingX.
backward compatibility Means you can still run your old program because we didnXt break any of the features or bugs it was relying on.
bareword A word sufficiently ambiguous to be deemed illegal under use strict subs. In the absence of that stricture, a bareword is treated as if quotes were around it.
base class A generic <B>objectB> type; that is, a <B>classB> from which other, more specific classes are derived genetically by <B>inheritanceB>. Also called a XsuperclassX by people who respect their ancestors.
big-endian From Swift: someone who eats eggs big end first. Also used of computers that store the most significant <B>byteB> of a word at a lower byte address than the least significant byte. Often considered superior to little-endian machines. See also <B>little-endianB>.
binary Having to do with numbers represented in base 2. That means thereXs basically two numbers: 0 and 1. Also used to describe a file of XnontextX, presumably because such a file makes full use of all the binary bits in its bytes. With the advent of <B>UnicodeB>, this distinction, already suspect, loses even more of its meaning.
binary operator An <B>operatorB> that takes two <B>operandsB>.
bind To assign a specific <B>network addressB> to a <B>socketB>.
bit An integer in the range from 0 to 1, inclusive. The smallest possible unit of information storage. An eighth of a <B>byteB> or of a dollar. (The term XPieces of EightX comes from being able to split the old Spanish dollar into 8 bits, each of which still counted for money. ThatXs why a 25- cent piece today is still Xtwo bitsX.)
bit shift The movement of bits left or right in a computer word, which has the effect of multiplying or dividing by a power of 2.
bit string A sequence of <B>bitsB> that is actually being thought of as a sequence of bits, for once.
bless In corporate life, to grant official approval to a thing, as in, XThe VP of Engineering has blessed our WebCruncher project.X Similarly, in Perl, to grant official approval to a <B>referentB> so that it can function as an <B>objectB>, such as a WebCruncher object. See the bless function in Camel chapter 27, XFunctionsX.
block What a <B>processB> does when it has to wait for something: XMy process blocked waiting for the disk.X As an unrelated noun, it refers to a large chunk of data, of a size that the <B>operating systemB> likes to deal with (normally a power of 2 such as 512 or 8192). Typically refers to a chunk of data thatXs coming from or going to a disk file.
BLOCK A syntactic construct consisting of a sequence of Perl <B>statementsB> that is delimited by braces. The if and while statements are defined in terms of BLOCKs, for instance. Sometimes we also say XblockX to mean a lexical scope; that is, a sequence of statements that acts like a BLOCK, such as within an eval or a file, even though the statements arenXt delimited by braces.
block buffering A method of making input and output efficient by passing one <B>blockB> at a time. By default, Perl does block buffering to disk files. See <B>bufferB> and <B>command bufferingB>.
Boolean A value that is either <B>trueB> or <B>falseB>.
Boolean context A special kind of <B>scalar contextB> used in conditionals to decide whether the <B>scalar valueB> returned by an expression is <B>trueB> or <B>falseB>. Does not evaluate as either a string or a number. See <B>contextB>.
breakpoint A spot in your program where youXve told the debugger to stop <B>executionB> so you can poke around and see whether anything is wrong yet.
broadcast To send a <B>datagramB> to multiple destinations simultaneously.
BSD A psychoactive drug, popular in the X80s, probably developed at UC Berkeley or thereabouts. Similar in many ways to the prescription-only medication called XSystem VX, but infinitely more useful. (Or, at least, more fun.) The full chemical name is XBerkeley Standard DistributionX.
bucket A location in a <B>hash tableB> containing (potentially) multiple entries whose keys XhashX to the same hash value according to its hash function. (As internal policy, you donXt have to worry about it unless youXre into internals, or policy.)
buffer A temporary holding location for data. Data that are <B>Block bufferingB> means that the data is passed on to its destination whenever the buffer is full. <B>Line bufferingB> means that itXs passed on whenever a complete line is received. <B>Command bufferingB> means that itXs passed every time you do a print command (or equivalent). If your output is unbuffered, the system processes it one byte at a time without the use of a holding area. This can be rather inefficient.
built-in A <B>functionB> that is predefined in the language. Even when hidden by <B>overridingB>, you can always get at a built- in function by <B>qualifyingB> its name with the CORE:: pseudopackage.
bundle A group of related modules on <B>CPANB>. (Also sometimes refers to a group of command-line switches grouped into one <B>switch clusterB>.)
byte A piece of data worth eight <B>bitsB> in most places.
bytecode A pidgin-like lingo spoken among Xdroids when they donXt wish to reveal their orientation (see <B>endianB>). Named after some similar languages spoken (for similar reasons) between compilers and interpreters in the late 20XX century. These languages are characterized by representing everything as a nonarchitecture-dependent sequence of bytes.


C A language beloved by many for its inside-out <B>typeB> definitions, inscrutable <B>precedenceB> rules, and heavy <B>overloadingB> of the function-call mechanism. (Well, actually, people first switched to C because they found lowercase identifiers easier to read than upper.) Perl is written in C, so itXs not surprising that Perl borrowed a few ideas from it.
cache A data repository. Instead of computing expensive answers several times, compute it once and save the result.
callback A <B>handlerB> that you register with some other part of your program in the hope that the other part of your program will <B>triggerB> your handler when some event of interest transpires.
call by reference An <B>argumentB>-passing mechanism in which the <B>formal argumentsB> refer directly to the <B>actual argumentsB>, and the <B>subroutineB> can change the actual arguments by changing the formal arguments. That is, the formal argument is an <B>aliasB> for the actual argument. See also <B>call by valueB>.
call by value An <B>argumentB>-passing mechanism in which the <B>formal argumentsB> refer to a copy of the <B>actual argumentsB>, and the <B>subroutineB> cannot change the actual arguments by changing the formal arguments. See also <B>call by referenceB>.
canonical Reduced to a standard form to facilitate comparison.
capture variables The variablesXsuch as $1 and $2, and %+ and %X Xthat hold the text remembered in a pattern match. See Camel chapter 5, XPattern MatchingX.
capturing The use of parentheses around a <B>subpatternB> in a <B>regular expressionB> to store the matched <B>substringB> as a <B>backreferenceB>. (Captured strings are also returned as a list in <B>list contextB>.) See Camel chapter 5, XPattern MatchingX.
cargo cult Copying and pasting code without understanding it, while superstitiously believing in its value. This term originated from preindustrial cultures dealing with the detritus of explorers and colonizers of technologically advanced cultures. See The Gods Must Be Crazy.
case A property of certain characters. Originally, typesetter stored capital letters in the upper of two cases and small letters in the lower one. Unicode recognizes three cases: <B>lowercaseB> (<B>character propertyB> \p{lower}), <B>titlecaseB> (\p{title}), and <B>uppercaseB> (\p{upper}). A fourth casemapping called <B>foldcaseB> is not itself a distinct case, but it is used internally to implement <B>casefoldingB>. Not all letters have case, and some nonletters have case.
casefolding Comparing or matching a string case-insensitively. In Perl, it is implemented with the /i pattern modifier, the fc function, and the \F double-quote translation escape.
casemapping The process of converting a string to one of the four Unicode <B>casemapsB>; in Perl, it is implemented with the fc, lc, ucfirst, and uc functions.
character The smallest individual element of a string. Computers store characters as integers, but Perl lets you operate on them as text. The integer used to represent a particular character is called that characterXs <B>codepointB>.
character class A square-bracketed list of characters used in a <B>regular expressionB> to indicate that any character of the set may occur at a given point. Loosely, any predefined set of characters so used.
character property A predefined <B>character classB> matchable by the \p or \P <B>metasymbolB>. <B>UnicodeB> defines hundreds of standard properties for every possible codepoint, and Perl defines a few of its own, too.
circumfix operator An <B>operatorB> that surrounds its <B>operandB>, like the angle operator, or parentheses, or a hug.
class A user-defined <B>typeB>, implemented in Perl via a <B>packageB> that provides (either directly or by inheritance) <B>methodsB> (that is, <B>subroutinesB>) to handle <B>instancesB> of the class (its <B>objectsB>). See also <B>inheritanceB>.
class method A <B>methodB> whose <B>invocantB> is a <B>packageB> name, not an <B>objectB> reference. A method associated with the class as a whole. Also see <B>instance methodB>.
client In networking, a <B>processB> that initiates contact with a <B>serverB> process in order to exchange data and perhaps receive a service.
closure An <B>anonymousB> subroutine that, when a reference to it is generated at runtime, keeps track of the identities of externally visible <B>lexical variablesB>, even after those lexical variables have supposedly gone out of <B>scopeB>. TheyXre called XclosuresX because this sort of behavior gives mathematicians a sense of closure.
cluster A parenthesized <B>subpatternB> used to group parts of a <B>regular expressionB> into a single <B>atomB>.
CODE The word returned by the ref function when you apply it to a reference to a subroutine. See also <B>CVB>.
code generator A system that writes code for you in a low-level language, such as code to implement the backend of a compiler. See <B>program generatorB>.
codepoint The integer a computer uses to represent a given character. ASCII codepoints are in the range 0 to 127; Unicode codepoints are in the range 0 to 0x1F_FFFF; and Perl codepoints are in the range 0 to 2XXX1 or 0 to 2XXX1, depending on your native integer size. In Perl Culture, sometimes called <B>ordinalsB>.
code subpattern A <B>regular expressionB> subpattern whose real purpose is to execute some Perl codeXfor example, the (?{...}) and (??{...}) subpatterns.
collating sequence The order into which <B>charactersB> sort. This is used by <B>stringB> comparison routines to decide, for example, where in this glossary to put Xcollating sequenceX.
co-maintainer A person with permissions to index a <B>namespaceB> in <B>PAUSEB>. Anyone can upload any namespace, but only primary and co-maintainers get their contributions indexed.
combining character Any character with the General Category of Combining Mark (\p{GC=M}), which may be spacing or nonspacing. Some are even invisible. A sequence of combining characters following a grapheme base character together make up a single user-visible character called a <B>graphemeB>. Most but not all diacritics are combining characters, and vice versa.
command In <B>shellB> programming, the syntactic combination of a program name and its arguments. More loosely, anything you type to a shell (a command interpreter) that starts it doing something. Even more loosely, a Perl <B>statementB>, which might start with a <B>labelB> and typically ends with a semicolon.
command buffering A mechanism in Perl that lets you store up the output of each Perl <B>commandB> and then flush it out as a single request to the <B>operating systemB>. ItXs enabled by setting the $| ($AUTOFLUSH) variable to a true value. ItXs used when you donXt want data sitting around, not going where itXs supposed to, which may happen because the default on a <B>fileB> or <B>pipeB> is to use <B>block bufferingB>.
command-line arguments The <B>valuesB> you supply along with a program name when you tell a <B>shellB> to execute a <B>commandB>. These values are passed to a Perl program through @ARGV.
command name The name of the program currently executing, as typed on the command line. In C, the <B>commandB> name is passed to the program as the first command-line argument. In Perl, it comes in separately as $0.
comment A remark that doesnXt affect the meaning of the program. In Perl, a comment is introduced by a # character and continues to the end of the line.
compilation unit The <B>fileB> (or <B>stringB>, in the case of eval) that is currently being <B>compiledB>.
compile The process of turning source code into a machine-usable form. See <B>compile phaseB>.
compile phase Any time before Perl starts running your main program. See also <B>run phaseB>. Compile phase is mostly spent in <B>compile timeB>, but may also be spent in <B>runtimeB> when BEGIN blocks, use or no declarations, or constant subexpressions are being evaluated. The startup and import code of any use declaration is also run during compile phase.
compiler Strictly speaking, a program that munches up another program and spits out yet another file containing the program in a Xmore executableX form, typically containing native machine instructions. The perl program is not a compiler by this definition, but it does contain a kind of compiler that takes a program and turns it into a more executable form (<B>syntax treesB>) within the perl process itself, which the <B>interpreterB> then interprets. There are, however, extension <B>modulesB> to get Perl to act more like a XrealX compiler. See Camel chapter 16, XCompilingX.
compile time The time when Perl is trying to make sense of your code, as opposed to when it thinks it knows what your code means and is merely trying to do what it thinks your code says to do, which is <B>runtimeB>.
composer A XconstructorX for a <B>referentB> that isnXt really an <B>objectB>, like an anonymous array or a hash (or a sonata, for that matter). For example, a pair of braces acts as a composer for a hash, and a pair of brackets acts as a composer for an array. See the section XCreating ReferencesX in Camel chapter 8, XReferencesX.
concatenation The process of gluing one catXs nose to another catXs tail. Also a similar operation on two <B>stringsB>.
conditional Something XiffyX. See <B>Boolean contextB>.
connection In telephony, the temporary electrical circuit between the callerXs and the calleeXs phone. In networking, the same kind of temporary circuit between a <B>clientB> and a <B>serverB>.
construct As a noun, a piece of syntax made up of smaller pieces. As a transitive verb, to create an <B>objectB> using a <B>constructorB>.
constructor Any <B>class methodB>, <B>instanceB>, or <B>subroutineB> that composes, initializes, blesses, and returns an <B>objectB>. Sometimes we use the term loosely to mean a <B>composerB>.
context The surroundings or environment. The context given by the surrounding code determines what kind of data a particular <B>expressionB> is expected to return. The three primary contexts are <B>list contextB>, <B>scalarB>, and <B>void contextB>. Scalar context is sometimes subdivided into <B>Boolean contextB>, <B>numeric contextB>, <B>string contextB>, and <B>void contextB>. ThereXs also a XdonXt careX context (which is dealt with in Camel chapter 2, XBits and PiecesX, if you care).
continuation The treatment of more than one physical <B>lineB> as a single logical line. <B>MakefileB> lines are continued by putting a backslash before the <B>newlineB>. Mail headers, as defined by RFC 822, are continued by putting a space or tab after the newline. In general, lines in Perl do not need any form of continuation mark, because <B>whitespaceB> (including newlines) is gleefully ignored. Usually.
core dump The corpse of a <B>processB>, in the form of a file left in the <B>working directoryB> of the process, usually as a result of certain kinds of fatal errors.
CPAN The Comprehensive Perl Archive Network. (See the Camel Preface and Camel chapter 19, XCPANX for details.)
C preprocessor The typical C compilerXs first pass, which processes lines beginning with # for conditional compilation and macro definition, and does various manipulations of the program text based on the current definitions. Also known as cpp(1).
cracker Someone who breaks security on computer systems. A cracker may be a true <B>hackerB> or only a <B>script kiddieB>.
currently selected output channel The last <B>filehandleB> that was designated with select(FILEHANDLE); STDOUT, if no filehandle has been selected.
current package The <B>packageB> in which the current statement is <B>compiledB>. Scan backward in the text of your program through the current <B>lexical scopeB> or any enclosing lexical scopes until you find a package declaration. ThatXs your current package name.
current working directory See <B>working directoryB>.
CV In academia, a curriculum vitae, a fancy kind of re\k:'|\n:u. In Perl, an internal Xcode valueX typedef holding a <B>subroutineB>. The CV type is a subclass of <B>SVB>.


dangling statement A bare, single <B>statementB>, without any braces, hanging off an if or while conditional. C allows them. Perl doesnXt.
datagram A packet of data, such as a <B>UDPB> message, that (from the viewpoint of the programs involved) can be sent independently over the network. (In fact, all packets are sent independently at the <B>IPB> level, but <B>streamB> protocols such as <B>TCPB> hide this from your program.)
data structure How your various pieces of data relate to each other and what shape they make when you put them all together, as in a rectangular table or a triangular tree.
data type A set of possible values, together with all the operations that know how to deal with those values. For example, a numeric data type has a certain set of numbers that you can work with, as well as various mathematical operations that you can do on the numbers, but would make little sense on, say, a string such as "Kilroy". Strings have their own operations, such as <B>concatenationB>. Compound types made of a number of smaller pieces generally have operations to compose and decompose them, and perhaps to rearrange them. <B>ObjectsB> that model things in the real world often have operations that correspond to real activities. For instance, if you model an elevator, your elevator object might have an open_door <B>methodB>.
DBM Stands for XDatabase ManagementX routines, a set of routines that emulate an <B>associative arrayB> using disk files. The routines use a dynamic hashing scheme to locate any entry with only two disk accesses. DBM files allow a Perl program to keep a persistent <B>hashB> across multiple invocations. You can tie your hash variables to various DBM implementations.
declaration An <B>assertionB> that states something exists and perhaps describes what itXs like, without giving any commitment as to how or where youXll use it. A declaration is like the part of your recipe that says, Xtwo cups flour, one large egg, four or five tadpolesXX See <B>statementB> for its opposite. Note that some declarations also function as statements. Subroutine declarations also act as definitions if a body is supplied.
declarator Something that tells your program what sort of variable youXd like. Perl doesnXt require you to declare variables, but you can use my, our, or state to denote that you want something other than the default.
decrement To subtract a value from a variable, as in Xdecrement $xX (meaning to remove 1 from its value) or Xdecrement $x by 3X.
default A <B>valueB> chosen for you if you donXt supply a value of your own.
defined Having a meaning. Perl thinks that some of the things people try to do are devoid of meaning; in particular, making use of variables that have never been given a <B>valueB> and performing certain operations on data that isnXt there. For example, if you try to read data past the end of a file, Perl will hand you back an undefined value. See also <B>falseB> and the defined entry in Camel chapter 27, XFunctionsX.
delimiter A <B>characterB> or <B>stringB> that sets bounds to an arbitrarily sized textual object, not to be confused with a <B>separatorB> or <B>terminatorB>. XTo delimitX really just means Xto surroundX or Xto encloseX (like these parentheses are doing).
dereference A fancy computer science term meaning Xto follow a <B>referenceB> to what it points toX. The XdeX part of it refers to the fact that youXre taking away one level of <B>indirectionB>.
derived class A <B>classB> that defines some of its <B>methodsB> in terms of a more generic class, called a <B>base classB>. Note that classes arenXt classified exclusively into base classes or derived classes: a class can function as both a derived class and a base class simultaneously, which is kind of classy.
descriptor See <B>file descriptorB>.
destroy To deallocate the memory of a <B>referentB> (first triggering its DESTROY method, if it has one).
destructor A special <B>methodB> that is called when an <B>objectB> is thinking about <B>destroyingB> itself. A Perl programXs DESTROY method doesnXt do the actual destruction; Perl just <B>triggersB> the method in case the <B>classB> wants to do any associated cleanup.
device A whiz-bang hardware gizmo (like a disk or tape drive or a modem or a joystick or a mouse) attached to your computer, which the <B>operating systemB> tries to make look like a <B>fileB> (or a bunch of files). Under Unix, these fake files tend to live in the /dev directory.
directive A <B>podB> directive. See Camel chapter 23, XPlain Old DocumentationX.
directory A special file that contains other files. Some <B>operating systemsB> call these XfoldersX, XdrawersX, XcataloguesX, or XcatalogsX.
directory handle A name that represents a particular instance of opening a directory to read it, until you close it. See the opendir function.
discipline Some people need this and some people avoid it. For Perl, itXs an old way to say <B>I/O layerB>.
dispatch To send something to its correct destination. Often used metaphorically to indicate a transfer of programmatic control to a destination selected algorithmically, often by lookup in a table of function <B>referencesB> or, in the case of object <B>methodsB>, by traversing the inheritance tree looking for the most specific definition for the method.
distribution A standard, bundled release of a system of software. The default usage implies source code is included. If that is not the case, it will be called a Xbinary-onlyX distribution.
dual-lived Some modules live both in the <B>Standard LibraryB> and on <B>CPANB>. These modules might be developed on two tracks as people modify either version. The trend currently is to untangle these situations.
dweomer An enchantment, illusion, phantasm, or jugglery. Said when PerlXs magical <B>dwimmerB> effects donXt do what you expect, but rather seem to be the product of arcane dweomercraft, sorcery, or wonder working. [From Middle English.]
dwimmer DWIM is an acronym for XDo What I MeanX, the principle that something should just do what you want it to do without an undue amount of fuss. A bit of code that does XdwimmingX is a XdwimmerX. Dwimming can require a great deal of behind-the-scenes magic, which (if it doesnXt stay properly behind the scenes) is called a <B>dweomerB> instead.
dynamic scoping Dynamic scoping works over a <B>dynamic scopeB>, making variables visible throughout the rest of the <B>blockB> in which they are first used and in any <B>subroutinesB> that are called by the rest of the block. Dynamically scoped variables can have their values temporarily changed (and implicitly restored later) by a local operator. (Compare <B>lexical scopingB>.) Used more loosely to mean how a subroutine that is in the middle of calling another subroutine XcontainsX that subroutine at <B>runtimeB>.


eclectic Derived from many sources. Some would say too many.
element A basic building block. When youXre talking about an <B>arrayB>, itXs one of the items that make up the array.
embedding When something is contained in something else, particularly when that might be considered surprising: XIXve embedded a complete Perl interpreter in my editor!X
empty subclass test The notion that an empty <B>derived classB> should behave exactly like its <B>base classB>.
encapsulation The veil of abstraction separating the <B>interfaceB> from the <B>implementationB> (whether enforced or not), which mandates that all access to an <B>objectB>Xs state be through <B>methodsB> alone.
endian See <B>little-endianB> and <B>big-endianB>.
en passant When you change a <B>valueB> as it is being copied. [From French Xin passingX, as in the exotic pawn-capturing maneuver in chess.]
environment The collective set of <B>environment variablesB> your <B>processB> inherits from its parent. Accessed via %ENV.
environment variable A mechanism by which some high-level agent such as a user can pass its preferences down to its future offspring (child <B>processesB>, grandchild processes, great-grandchild processes, and so on). Each environment variable is a <B>keyB>/<B>valueB> pair, like one entry in a <B>hashB>.
EOF End of File. Sometimes used metaphorically as the terminating string of a <B>here documentB>.
errno The error number returned by a <B>syscallB> when it fails. Perl refers to the error by the name $! (or $OS_ERROR if you use the English module).
error See <B>exceptionB> or <B>fatal errorB>.
escape sequence See <B>metasymbolB>.
exception A fancy term for an error. See <B>fatal errorB>.
exception handling The way a program responds to an error. The exception-handling mechanism in Perl is the eval operator.
exec To throw away the current <B>processB>Xs program and replace it with another, without exiting the process or relinquishing any resources held (apart from the old memory image).
executable file A <B>fileB> that is specially marked to tell the <B>operating systemB> that itXs okay to run this file as a program. Usually shortened to XexecutableX.
execute To run a <B>programB> or <B>subroutineB>. (Has nothing to do with the kill built-in, unless youXre trying to run a <B>signal handlerB>.)
execute bit The special mark that tells the operating system it can run this program. There are actually three execute bits under Unix, and which bit gets used depends on whether you own the file singularly, collectively, or not at all.
exit status See <B>statusB>.
exploit Used as a noun in this case, this refers to a known way to compromise a program to get it to do something the author didnXt intend. Your task is to write unexploitable programs.
export To make symbols from a <B>moduleB> available for <B>importB> by other modules.
expression Anything you can legally say in a spot where a <B>valueB> is required. Typically composed of <B>literalsB>, <B>variablesB>, <B>operatorsB>, <B>functionsB>, and <B>subroutineB> calls, not necessarily in that order.
extension A Perl module that also pulls in <B>compiledB> C or C++ code. More generally, any experimental option that can be <B>compiledB> into Perl, such as multithreading.


false In Perl, any value that would look like "" or "0" if evaluated in a string context. Since undefined values evaluate to "", all undefined values are false, but not all false values are undefined.
FAQ Frequently Asked Question (although not necessarily frequently answered, especially if the answer appears in the Perl FAQ shipped standard with Perl).
fatal error An uncaught <B>exceptionB>, which causes termination of the <B>processB> after printing a message on your <B>standard errorB> stream. Errors that happen inside an eval are not fatal. Instead, the eval terminates after placing the exception message in the $@ ($EVAL_ERROR) variable. You can try to provoke a fatal error with the die operator (known as throwing or raising an exception), but this may be caught by a dynamically enclosing eval. If not caught, the die becomes a fatal error.
feeping creaturism A spoonerism of Xcreeping featurismX, noting the biological urge to add just one more feature to a program.
field A single piece of numeric or string data that is part of a longer <B>stringB>, <B>recordB>, or <B>lineB>. Variable-width fields are usually split up by <B>separatorsB> (so use split to extract the fields), while fixed-width fields are usually at fixed positions (so use unpack). <B>Instance variablesB> are also known as XfieldsX.
FIFO First In, First Out. See also <B>LIFOB>. Also a nickname for a <B>named pipeB>.
file A named collection of data, usually stored on disk in a <B>directoryB> in a <B>filesystemB>. Roughly like a document, if youXre into office metaphors. In modern filesystems, you can actually give a file more than one name. Some files have special properties, like directories and devices.
file descriptor The little number the <B>operating systemB> uses to keep track of which opened <B>fileB> youXre talking about. Perl hides the file descriptor inside a <B>standard I/OB> stream and then attaches the stream to a <B>filehandleB>.
fileglob A XwildcardX match on <B>filenamesB>. See the glob function.
filehandle An identifier (not necessarily related to the real name of a file) that represents a particular instance of opening a file, until you close it. If youXre going to open and close several different files in succession, itXs fine to open each of them with the same filehandle, so you donXt have to write out separate code to process each file.
filename One name for a file. This name is listed in a <B>directoryB>. You can use it in an open to tell the <B>operating systemB> exactly which file you want to open, and associate the file with a <B>filehandleB>, which will carry the subsequent identity of that file in your program, until you close it.
filesystem A set of <B>directoriesB> and <B>filesB> residing on a partition of the disk. Sometimes known as a XpartitionX. You can change the fileXs name or even move a file around from directory to directory within a filesystem without actually moving the file itself, at least under Unix.
file test operator A built-in unary operator that you use to determine whether something is <B>trueB> about a file, such as Xo $filename to test whether youXre the owner of the file.
filter A program designed to take a <B>streamB> of input and transform it into a stream of output.
first-come The first <B>PAUSEB> author to upload a <B>namespaceB> automatically becomes the <B>primary maintainerB> for that namespace. The Xfirst comeX permissions distinguish a <B>primary maintainerB> who was assigned that role from one who received it automatically.
flag We tend to avoid this term because it means so many things. It may mean a command-line <B>switchB> that takes no argument itself (such as PerlXs Xn and Xp flags) or, less frequently, a single-bit indicator (such as the O_CREAT and O_EXCL flags used in sysopen). Sometimes informally used to refer to certain regex modifiers.
floating point A method of storing numbers in Xscientific notationX, such that the precision of the number is independent of its magnitude (the decimal point XfloatsX). Perl does its numeric work with floating-point numbers (sometimes called XfloatsX) when it canXt get away with using <B>integersB>. Floating-point numbers are mere approximations of real numbers.
flush The act of emptying a <B>bufferB>, often before itXs full.
FMTEYEWTK Far More Than Everything You Ever Wanted To Know. An exhaustive treatise on one narrow topic, something of a super-<B>FAQB>. See Tom for far more.
foldcase The casemap used in Unicode when comparing or matching without regard to case. Comparing lower-, title-, or uppercase are all unreliable due to UnicodeXs complex, one-to-many case mappings. Foldcase is a <B>lowercaseB> variant (using a partially decomposed <B>normalizationB> form for certain codepoints) created specifically to resolve this.
fork To create a child <B>processB> identical to the parent process at its moment of conception, at least until it gets ideas of its own. A thread with protected memory.
formal arguments The generic names by which a <B>subroutineB> knows its <B>argumentsB>. In many languages, formal arguments are always given individual names; in Perl, the formal arguments are just the elements of an array. The formal arguments to a Perl program are $ARGV[0], $ARGV[1], and so on. Similarly, the formal arguments to a Perl subroutine are $_[0], $_[1], and so on. You may give the arguments individual names by assigning the values to a my list. See also <B>actual argumentsB>.
format A specification of how many spaces and digits and things to put somewhere so that whatever youXre printing comes out nice and pretty.
freely available Means you donXt have to pay money to get it, but the copyright on it may still belong to someone else (like Larry).
freely redistributable Means youXre not in legal trouble if you give a bootleg copy of it to your friends and we find out about it. In fact, weXd rather you gave a copy to all your friends.
freeware Historically, any software that you give away, particularly if you make the source code available as well. Now often called <B>open source softwareB>. Recently there has been a trend to use the term in contradistinction to <B>open source softwareB>, to refer only to free software released under the Free Software FoundationXs GPL (General Public License), but this is difficult to justify etymologically.
function Mathematically, a mapping of each of a set of input values to a particular output value. In computers, refers to a <B>subroutineB> or <B>operatorB> that returns a <B>valueB>. It may or may not have input values (called <B>argumentsB>).
funny character Someone like Larry, or one of his peculiar friends. Also refers to the strange prefixes that Perl requires as noun markers on its variables.


garbage collection A misnamed featureXit should be called, Xexpecting your mother to pick up after youX. Strictly speaking, Perl doesnXt do this, but it relies on a reference-counting mechanism to keep things tidy. However, we rarely speak strictly and will often refer to the reference-counting scheme as a form of garbage collection. (If itXs any comfort, when your interpreter exits, a XrealX garbage collector runs to make sure everything is cleaned up if youXve been messy with circular references and such.)
GID Group IDXin Unix, the numeric group ID that the <B>operating systemB> uses to identify you and members of your <B>groupB>.
glob Strictly, the shellXs * character, which will match a XglobX of characters when youXre trying to generate a list of filenames. Loosely, the act of using globs and similar symbols to do pattern matching. See also <B>fileglobB> and <B>typeglobB>.
global Something you can see from anywhere, usually used of <B>variablesB> and <B>subroutinesB> that are visible everywhere in your program. In Perl, only certain special variables are truly globalXmost variables (and all subroutines) exist only in the current <B>packageB>. Global variables can be declared with our. See XGlobal DeclarationsX in Camel chapter 4, XStatements and DeclarationsX.
global destruction The <B>garbage collectionB> of globals (and the running of any associated object destructors) that takes place when a Perl <B>interpreterB> is being shut down. Global destruction should not be confused with the Apocalypse, except perhaps when it should.
glue language A language such as Perl that is good at hooking things together that werenXt intended to be hooked together.
granularity The size of the pieces youXre dealing with, mentally speaking.
grapheme A graphene is an allotrope of carbon arranged in a hexagonal crystal lattice one atom thick. A <B>graphemeB>, or more fully, a grapheme cluster string is a single user-visible <B>characterB>, which may in turn be several characters (<B>codepointsB>) long. For example, a carriage return plus a line feed is a single grapheme but two characters, while a XXX is a single grapheme but one, two, or even three characters, depending on <B>normalizationB>.
greedy A <B>subpatternB> whose <B>quantifierB> wants to match as many things as possible.
grep Originally from the old Unix editor command for XGlobally search for a Regular Expression and Print itX, now used in the general sense of any kind of search, especially text searches. Perl has a built-in grep function that searches a list for elements matching any given criterion, whereas the <B>grepB>(1) program searches for lines matching a <B>regular expressionB> in one or more files.
group A set of users of which you are a member. In some operating systems (like Unix), you can give certain file access permissions to other members of your group.
GV An internal Xglob valueX typedef, holding a <B>typeglobB>. The GV type is a subclass of <B>SVB>.


hacker Someone who is brilliantly persistent in solving technical problems, whether these involve golfing, fighting orcs, or programming. Hacker is a neutral term, morally speaking. Good hackers are not to be confused with evil <B>crackersB> or clueless <B>script kiddiesB>. If you confuse them, we will presume that you are either evil or clueless.
handler A <B>subroutineB> or <B>methodB> that Perl calls when your program needs to respond to some internal event, such as a <B>signalB>, or an encounter with an operator subject to <B>operator overloadingB>. See also <B>callbackB>.
hard reference A <B>scalarB> <B>valueB> containing the actual address of a <B>referentB>, such that the referentXs <B>referenceB> count accounts for it. (Some hard references are held internally, such as the implicit reference from one of a <B>typeglobB>Xs variable slots to its corresponding referent.) A hard reference is different from a <B>symbolic referenceB>.
hash An unordered association of <B>keyB>/<B>valueB> pairs, stored such that you can easily use a string <B>keyB> to look up its associated data <B>valueB>. This glossary is like a hash, where the word to be defined is the key and the definition is the value. A hash is also sometimes septisyllabically called an Xassociative arrayX, which is a pretty good reason for simply calling it a XhashX instead.
hash table A data structure used internally by Perl for implementing associative arrays (hashes) efficiently. See also <B>bucketB>.
header file A file containing certain required definitions that you must include XaheadX of the rest of your program to do certain obscure operations. A C header file has a .h extension. Perl doesnXt really have header files, though historically Perl has sometimes used translated .h files with a .ph extension. See require in Camel chapter 27, XFunctionsX. (Header files have been superseded by the <B>moduleB> mechanism.)
here document So called because of a similar construct in <B>shellsB> that pretends that the <B>linesB> following the <B>commandB> are a separate <B>fileB> to be fed to the command, up to some terminating string. In Perl, however, itXs just a fancy form of quoting.
hexadecimal A number in base 16, XhexX for short. The digits for 10 through 15 are customarily represented by the letters a through f. Hexadecimal constants in Perl start with 0x. See also the hex function in Camel chapter 27, XFunctionsX.
home directory The directory you are put into when you log in. On a Unix system, the name is often placed into $ENV{HOME} or $ENV{LOGDIR} by login, but you can also find it with (getpwuid($<))[7]. (Some platforms do not have a concept of a home directory.)
host The computer on which a program or other data resides.
hubris Excessive pride, the sort of thing for which Zeus zaps you. Also the quality that makes you write (and maintain) programs that other people wonXt want to say bad things about. Hence, the third great virtue of a programmer. See also <B>lazinessB> and <B>impatienceB>.
HV Short for a Xhash valueX typedef, which holds PerlXs internal representation of a hash. The HV type is a subclass of <B>SVB>.


identifier A legally formed name for most anything in which a computer program might be interested. Many languages (including Perl) allow identifiers to start with an alphabetic character, and then contain alphabetics and digits. Perl also allows connector punctuation like the underscore character wherever it allows alphabetics. (Perl also has more complicated names, like <B>qualifiedB> names.)
impatience The anger you feel when the computer is being lazy. This makes you write programs that donXt just react to your needs, but actually anticipate them. Or at least that pretend to. Hence, the second great virtue of a programmer. See also <B>lazinessB> and <B>hubrisB>.
implementation How a piece of code actually goes about doing its job. Users of the code should not count on implementation details staying the same unless they are part of the published <B>interfaceB>.
import To gain access to symbols that are exported from another module. See use in Camel chapter 27, XFunctionsX.
increment To increase the value of something by 1 (or by some other number, if so specified).
indexing In olden days, the act of looking up a <B>keyB> in an actual index (such as a phone book). But now it’s merely the act of using any kind of key or position to find the corresponding <B>valueB>, even if no index is involved. Things have degenerated to the point that PerlXs index function merely locates the position (index) of one string in another.
indirect filehandle An <B>expressionB> that evaluates to something that can be used as a <B>filehandleB>: a <B>stringB> (filehandle name), a <B>typeglobB>, a typeglob <B>referenceB>, or a low-level <B>IOB> object.
indirection If something in a program isnXt the value youXre looking for but indicates where the value is, thatXs indirection. This can be done with either <B>symbolic referencesB> or <B>hardB>.
indirect object In English grammar, a short noun phrase between a verb and its direct object indicating the beneficiary or recipient of the action. In Perl, print STDOUT "$foo\n"; can be understood as Xverb indirect-object objectX, where STDOUT is the recipient of the print action, and "$foo" is the object being printed. Similarly, when invoking a <B>methodB>, you might place the invocant in the dative slot between the method and its arguments:

    $gollum = new Pathetic::Creature "Smeágol";
    give $gollum "Fisssssh!";
    give $gollum "Precious!";

indirect object slot The syntactic position falling between a method call and its arguments when using the indirect object invocation syntax. (The slot is distinguished by the absence of a comma between it and the next argument.) STDERR is in the indirect object slot here:

    print STDERR "Awake! Awake! Fear, Fire, Foes! Awake!\n";

infix An <B>operatorB> that comes in between its <B>operandsB>, such as multiplication in 24 * 7.
inheritance What you get from your ancestors, genetically or otherwise. If you happen to be a <B>classB>, your ancestors are called <B>base classesB> and your descendants are called <B>derived classesB>. See <B>single inheritanceB> and <B>multiple inheritanceB>.
instance Short for Xan instance of a classX, meaning an <B>objectB> of that <B>classB>.
instance data See <B>instance variableB>.
instance method A <B>methodB> of an <B>objectB>, as opposed to a <B>class methodB>.

A <B>methodB> whose <B>invocantB> is an <B>objectB>, not a <B>packageB> name. Every object of a class shares all the methods of that class, so an instance method applies to all instances of the class, rather than applying to a particular instance. Also see <B>class methodB>.

instance variable An <B>attributeB> of an <B>objectB>; data stored with the particular object rather than with the class as a whole.
integer A number with no fractional (decimal) part. A counting number, like 1, 2, 3, and so on, but including 0 and the negatives.
interface The services a piece of code promises to provide forever, in contrast to its <B>implementationB>, which it should feel free to change whenever it likes.
interpolation The insertion of a scalar or list value somewhere in the middle of another value, such that it appears to have been there all along. In Perl, variable interpolation happens in double-quoted strings and patterns, and list interpolation occurs when constructing the list of values to pass to a list operator or other such construct that takes a LIST.
interpreter Strictly speaking, a program that reads a second program and does what the second program says directly without turning the program into a different form first, which is what <B>compilersB> do. Perl is not an interpreter by this definition, because it contains a kind of compiler that takes a program and turns it into a more executable form (<B>syntax treesB>) within the perl process itself, which the Perl <B>runtimeB> system then interprets.
invocant The agent on whose behalf a <B>methodB> is invoked. In a <B>classB> method, the invocant is a package name. In an <B>instanceB> method, the invocant is an object reference.
invocation The act of calling up a deity, daemon, program, method, subroutine, or function to get it to do what you think itXs supposed to do. We usually XcallX subroutines but XinvokeX methods, since it sounds cooler.
I/O Input from, or output to, a <B>fileB> or <B>deviceB>.
IO An internal I/O object. Can also mean <B>indirect objectB>.
I/O layer One of the filters between the data and what you get as input or what you end up with as output.
IPA India Pale Ale. Also the International Phonetic Alphabet, the standard alphabet used for phonetic notation worldwide. Draws heavily on Unicode, including many combining characters.
IP Internet Protocol, or Intellectual Property.
IPC Interprocess Communication.
is-a A relationship between two <B>objectsB> in which one object is considered to be a more specific version of the other, generic object: XA camel is a mammal.X Since the generic object really only exists in a Platonic sense, we usually add a little abstraction to the notion of objects and think of the relationship as being between a generic <B>base classB> and a specific <B>derived classB>. Oddly enough, Platonic classes donXt always have Platonic relationshipsXsee <B>inheritanceB>.
iteration Doing something repeatedly.
iterator A special programming gizmo that keeps track of where you are in something that youXre trying to iterate over. The foreach loop in Perl contains an iterator; so does a hash, allowing you to each through it.
IV The integer four, not to be confused with six, TomXs favorite editor. IV also means an internal Integer Value of the type a <B>scalarB> can hold, not to be confused with an <B>NVB>.


JAPH XJust Another Perl HackerX, a clever but cryptic bit of Perl code that, when executed, evaluates to that string. Often used to illustrate a particular Perl feature, and something of an ongoing Obfuscated Perl Contest seen in USENET signatures.


key The string index to a <B>hashB>, used to look up the <B>valueB> associated with that key.
keyword See <B>reserved wordsB>.


label A name you give to a <B>statementB> so that you can talk about that statement elsewhere in the program.
laziness The quality that makes you go to great effort to reduce overall energy expenditure. It makes you write labor-saving programs that other people will find useful, and then document what you wrote so you donXt have to answer so many questions about it. Hence, the first great virtue of a programmer. Also hence, this book. See also <B>impatienceB> and <B>hubrisB>.
leftmost longest The preference of the <B>regular expressionB> engine to match the leftmost occurrence of a <B>patternB>, then given a position at which a match will occur, the preference for the longest match (presuming the use of a <B>greedyB> quantifier). See Camel chapter 5, XPattern MatchingX for much more on this subject.
left shift A <B>bit shiftB> that multiplies the number by some power of 2.
lexeme Fancy term for a <B>tokenB>.
lexer Fancy term for a <B>tokenerB>.
lexical analysis Fancy term for <B>tokenizingB>.
lexical scoping Looking at your Oxford English Dictionary through a microscope. (Also known as <B>static scopingB>, because dictionaries donXt change very fast.) Similarly, looking at variables stored in a private dictionary (namespace) for each scope, which are visible only from their point of declaration down to the end of the lexical scope in which they are declared. XSyn. <B>static scopingB>. XAnt. <B>dynamic scopingB>.
lexical variable A <B>variableB> subject to <B>lexical scopingB>, declared by my. Often just called a XlexicalX. (The our declaration declares a lexically scoped name for a global variable, which is not itself a lexical variable.)
library Generally, a collection of procedures. In ancient days, referred to a collection of subroutines in a .pl file. In modern times, refers more often to the entire collection of Perl <B>modulesB> on your system.
LIFO Last In, First Out. See also <B>FIFOB>. A LIFO is usually called a <B>stackB>.
line In Unix, a sequence of zero or more nonnewline characters terminated with a <B>newlineB> character. On non-Unix machines, this is emulated by the C library even if the underlying <B>operating systemB> has different ideas.
linebreak A <B>graphemeB> consisting of either a carriage return followed by a line feed or any character with the Unicode Vertical Space <B>character propertyB>.
line buffering Used by a <B>standard I/OB> output stream that flushes its <B>bufferB> after every <B>newlineB>. Many standard I/O libraries automatically set up line buffering on output that is going to the terminal.
line number The number of lines read previous to this one, plus 1. Perl keeps a separate line number for each source or input file it opens. The current source fileXs line number is represented by __LINE__. The current input line number (for the file that was most recently read via <FH>) is represented by the $. ($INPUT_LINE_NUMBER) variable. Many error messages report both values, if available.
link Used as a noun, a name in a <B>directoryB> that represents a <B>fileB>. A given file can have multiple links to it. ItXs like having the same phone number listed in the phone directory under different names. As a verb, to resolve a partially <B>compiledB> fileXs unresolved symbols into a (nearly) executable image. Linking can generally be static or dynamic, which has nothing to do with static or dynamic scoping.
LIST A syntactic construct representing a comma- separated list of expressions, evaluated to produce a <B>list valueB>. Each <B>expressionB> in a LIST is evaluated in <B>list contextB> and interpolated into the list value.
list An ordered set of scalar values.
list context The situation in which an <B>expressionB> is expected by its surroundings (the code calling it) to return a list of values rather than a single value. Functions that want a LIST of arguments tell those arguments that they should produce a list value. See also <B>contextB>.
list operator An <B>operatorB> that does something with a list of values, such as join or grep. Usually used for named built-in operators (such as print, unlink, and system) that do not require parentheses around their <B>argumentB> list.
list value An unnamed list of temporary scalar values that may be passed around within a program from any list-generating function to any function or construct that provides a <B>list contextB>.
literal A token in a programming language, such as a number or <B>stringB>, that gives you an actual <B>valueB> instead of merely representing possible values as a <B>variableB> does.
little-endian From Swift: someone who eats eggs little end first. Also used of computers that store the least significant <B>byteB> of a word at a lower byte address than the most significant byte. Often considered superior to big-endian machines. See also <B>big-endianB>.
local Not meaning the same thing everywhere. A global variable in Perl can be localized inside a <B>dynamic scopeB> via the local operator.
logical operator Symbols representing the concepts XandX, XorX, XxorX, and XnotX.
lookahead An <B>assertionB> that peeks at the string to the right of the current match location.
lookbehind An <B>assertionB> that peeks at the string to the left of the current match location.
loop A construct that performs something repeatedly, like a roller coaster.
loop control statement Any statement within the body of a loop that can make a loop prematurely stop looping or skip an <B>iterationB>. Generally, you shouldnXt try this on roller coasters.
loop label A kind of key or name attached to a loop (or roller coaster) so that loop control statements can talk about which loop they want to control.
lowercase In Unicode, not just characters with the General Category of Lowercase Letter, but any character with the Lowercase property, including Modifier Letters, Letter Numbers, some Other Symbols, and one Combining Mark.
lvaluable Able to serve as an <B>lvalueB>.
lvalue Term used by language lawyers for a storage location you can assign a new <B>valueB> to, such as a <B>variableB> or an element of an <B>arrayB>. The XlX is short for XleftX, as in the left side of an assignment, a typical place for lvalues. An <B>lvaluableB> function or expression is one to which a value may be assigned, as in pos($x) = 10.
lvalue modifier An adjectival pseudofunction that warps the meaning of an <B>lvalueB> in some declarative fashion. Currently there are three lvalue modifiers: my, our, and local.


magic Technically speaking, any extra semantics attached to a variable such as $!, $0, %ENV, or %SIG, or to any tied variable. Magical things happen when you diddle those variables.
magical increment An <B>incrementB> operator that knows how to bump up ASCII alphabetics as well as numbers.
magical variables Special variables that have side effects when you access them or assign to them. For example, in Perl, changing elements of the %ENV array also changes the corresponding environment variables that subprocesses will use. Reading the $! variable gives you the current system error number or message.
Makefile A file that controls the compilation of a program. Perl programs donXt usually need a <B>MakefileB> because the Perl compiler has plenty of self-control.
man The Unix program that displays online documentation (manual pages) for you.
manpage A XpageX from the manuals, typically accessed via the man(1) command. A manpage contains a SYNOPSIS, a DESCRIPTION, a list of BUGS, and so on, and is typically longer than a page. There are manpages documenting <B>commandsB>, <B>syscallsB>, <B>libraryB> <B>functionsB>, <B>devicesB>, <B>protocolsB>, <B>filesB>, and such. In this book, we call any piece of standard Perl documentation (like perlop or perldelta) a manpage, no matter what format itXs installed in on your system.
matching See <B>pattern matchingB>.
member data See <B>instance variableB>.
memory This always means your main memory, not your disk. Clouding the issue is the fact that your machine may implement <B>virtualB> memory; that is, it will pretend that it has more memory than it really does, and itXll use disk space to hold inactive bits. This can make it seem like you have a little more memory than you really do, but itXs not a substitute for real memory. The best thing that can be said about virtual memory is that it lets your performance degrade gradually rather than suddenly when you run out of real memory. But your program can die when you run out of virtual memory, tooXif you havenXt thrashed your disk to death first.
metacharacter A <B>characterB> that is not supposed to be treated normally. Which characters are to be treated specially as metacharacters varies greatly from context to context. Your <B>shellB> will have certain metacharacters, double-quoted Perl <B>stringsB> have other metacharacters, and <B>regular expressionB> patterns have all the double-quote metacharacters plus some extra ones of their own.
metasymbol Something weXd call a <B>metacharacterB> except that itXs a sequence of more than one character. Generally, the first character in the sequence must be a true metacharacter to get the other characters in the metasymbol to misbehave along with it.
method A kind of action that an <B>objectB> can take if you tell it to. See Camel chapter 12, XObjectsX.
method resolution order The path Perl takes through @INC. By default, this is a double depth first search, once looking for defined methods and once for AUTOLOAD. However, Perl lets you configure this with mro.
minicpan A CPAN mirror that includes just the latest versions for each distribution, probably created with CPAN::Mini. See Camel chapter 19, XCPANX.
minimalism The belief that Xsmall is beautifulX. Paradoxically, if you say something in a small language, it turns out big, and if you say it in a big language, it turns out small. Go figure.
mode In the context of the stat(2) syscall, refers to the field holding the <B>permission bitsB> and the type of the <B>fileB>.
modifier See <B>statement modifierB>, <B>regular expressionB>, and <B>lvalueB>, not necessarily in that order.
module A <B>fileB> that defines a <B>packageB> of (almost) the same name, which can either <B>exportB> symbols or function as an <B>objectB> class. (A moduleXs main .pm file may also load in other files in support of the module.) See the use built-in.
modulus An integer divisor when youXre interested in the remainder instead of the quotient.
mojibake When you speak one language and the computer thinks youXre speaking another. YouXll see odd translations when you send UTFX8, for instance, but the computer thinks you sent Latin-1, showing all sorts of weird characters instead. The term is written XXXXXXin Japanese and means Xcharacter rotX, an apt description. Pronounced [modXibake] in standard <B>IPAB> phonetics, or approximately Xmoh-jee-bah-kehX.
monger Short for one member of <B>Perl mongersB>, a purveyor of Perl.
mortal A temporary value scheduled to die when the current statement finishes.
mro See <B>method resolution orderB>.
multidimensional array An array with multiple subscripts for finding a single element. Perl implements these using <B>referencesB>Xsee Camel chapter 9, XData StructuresX.
multiple inheritance The features you got from your mother and father, mixed together unpredictably. (See also <B>inheritanceB> and <B>single inheritanceB>.) In computer languages (including Perl), it is the notion that a given class may have multiple direct ancestors or <B>base classesB>.


named pipe A <B>pipeB> with a name embedded in the <B>filesystemB> so that it can be accessed by two unrelated <B>processesB>.
namespace A domain of names. You neednXt worry about whether the names in one such domain have been used in another. See <B>packageB>.
NaN Not a number. The value Perl uses for certain invalid or inexpressible floating-point operations.
network address The most important attribute of a socket, like your telephoneXs telephone number. Typically an IP address. See also <B>portB>.
newline A single character that represents the end of a line, with the ASCII value of 012 octal under Unix (but 015 on a Mac), and represented by \n in Perl strings. For Windows machines writing text files, and for certain physical devices like terminals, the single newline gets automatically translated by your C library into a line feed and a carriage return, but normally, no translation is done.
NFS Network File System, which allows you to mount a remote filesystem as if it were local.
normalization Converting a text string into an alternate but equivalent <B>canonicalB> (or compatible) representation that can then be compared for equivalence. Unicode recognizes four different normalization forms: NFD, NFC, NFKD, and NFKC.
null character A character with the numeric value of zero. ItXs used by C to terminate strings, but Perl allows strings to contain a null.
null list A <B>list valueB> with zero elements, represented in Perl by ().
null string A <B>stringB> containing no characters, not to be confused with a string containing a <B>null characterB>, which has a positive length and is <B>trueB>.
numeric context The situation in which an expression is expected by its surroundings (the code calling it) to return a number. See also <B>contextB> and <B>string contextB>.
numification (Sometimes spelled nummification and nummify.) Perl lingo for implicit conversion into a number; the related verb is numify. Numification is intended to rhyme with mummification, and numify with mummify. It is unrelated to English numen, numina, numinous. We originally forgot the extra m a long time ago, and some people got used to our funny spelling, and so just as with HTTP_REFERERXs own missing letter, our weird spelling has stuck around.
NV Short for Nevada, no part of which will ever be confused with civilization. NV also means an internal floating- point Numeric Value of the type a <B>scalarB> can hold, not to be confused with an <B>IVB>.
nybble Half a <B>byteB>, equivalent to one <B>hexadecimalB> digit, and worth four <B>bitsB>.


object An <B>instanceB> of a <B>classB>. Something that XknowsX what user-defined type (class) it is, and what it can do because of what class it is. Your program can request an object to do things, but the object gets to decide whether it wants to do them or not. Some objects are more accommodating than others.
octal A number in base 8. Only the digits 0 through 7 are allowed. Octal constants in Perl start with 0, as in 013. See also the oct function.
offset How many things you have to skip over when moving from the beginning of a string or array to a specific position within it. Thus, the minimum offset is zero, not one, because you donXt skip anything to get to the first item.
one-liner An entire computer program crammed into one line of text.
open source software Programs for which the source code is freely available and freely redistributable, with no commercial strings attached. For a more detailed definition, see <>.
operand An <B>expressionB> that yields a <B>valueB> that an <B>operatorB> operates on. See also <B>precedenceB>.
operating system A special program that runs on the bare machine and hides the gory details of managing <B>processesB> and <B>devicesB>. Usually used in a looser sense to indicate a particular culture of programming. The loose sense can be used at varying levels of specificity. At one extreme, you might say that all versions of Unix and Unix-lookalikes are the same operating system (upsetting many people, especially lawyers and other advocates). At the other extreme, you could say this particular version of this particular vendorXs operating system is different from any other version of this or any other vendorXs operating system. Perl is much more portable across operating systems than many other languages. See also <B>architectureB> and <B>platformB>.
operator A gizmo that transforms some number of input values to some number of output values, often built into a language with a special syntax or symbol. A given operator may have specific expectations about what <B>typesB> of data you give as its arguments (<B>operandsB>) and what type of data you want back from it.
operator overloading A kind of <B>overloadingB> that you can do on built-in <B>operatorsB> to make them work on <B>objectsB> as if the objects were ordinary scalar values, but with the actual semantics supplied by the object class. This is set up with the overload <B>pragmaB>Xsee Camel chapter 13, XOverloadingX.
options See either <B>switchesB> or <B>regular expression modifiersB>.
ordinal An abstract characterXs integer value. Same thing as <B>codepointB>.
overloading Giving additional meanings to a symbol or construct. Actually, all languages do overloading to one extent or another, since people are good at figuring out things from <B>contextB>.
overriding Hiding or invalidating some other definition of the same name. (Not to be confused with <B>overloadingB>, which adds definitions that must be disambiguated some other way.) To confuse the issue further, we use the word with two overloaded definitions: to describe how you can define your own <B>subroutineB> to hide a built-in <B>functionB> of the same name (see the section XOverriding Built-in FunctionsX in Camel chapter 11, XModulesX), and to describe how you can define a replacement <B>methodB> in a <B>derived classB> to hide a <B>base classB>Xs method of the same name (see Camel chapter 12, XObjectsX).
owner The one user (apart from the superuser) who has absolute control over a <B>fileB>. A file may also have a <B>groupB> of users who may exercise joint ownership if the real owner permits it. See <B>permission bitsB>.


package A <B>namespaceB> for global <B>variablesB>, <B>subroutinesB>, and the like, such that they can be kept separate from like-named <B>symbolsB> in other namespaces. In a sense, only the package is global, since the symbols in the packageXs symbol table are only accessible from code <B>compiledB> outside the package by naming the package. But in another sense, all package symbols are also globalsXtheyXre just well-organized globals.
pad Short for <B>scratchpadB>.
parameter See <B>argumentB>.
parent class See <B>base classB>.
parse tree See <B>syntax treeB>.
parsing The subtle but sometimes brutal art of attempting to turn your possibly malformed program into a valid <B>syntax treeB>.
patch To fix by applying one, as it were. In the realm of hackerdom, a listing of the differences between two versions of a program as might be applied by the <B>patchB>(1) program when you want to fix a bug or upgrade your old version.
PATH The list of <B>directoriesB> the system searches to find a program you want to <B>executeB>. The list is stored as one of your <B>environment variablesB>, accessible in Perl as $ENV{PATH}.
pathname A fully qualified filename such as /usr/bin/perl. Sometimes confused with PATH.
pattern A template used in <B>pattern matchingB>.
pattern matching Taking a pattern, usually a <B>regular expressionB>, and trying the pattern various ways on a string to see whether thereXs any way to make it fit. Often used to pick interesting tidbits out of a file.
PAUSE The Perl Authors Upload SErver (<>), the gateway for <B>modulesB> on their way to <B>CPANB>.
Perl mongers A Perl user group, taking the form of its name from the New York Perl mongers, the first Perl user group. Find one near you at <>.
permission bits Bits that the <B>ownerB> of a file sets or unsets to allow or disallow access to other people. These flag bits are part of the <B>modeB> word returned by the stat built-in when you ask about a file. On Unix systems, you can check the ls(1) manpage for more information.
Pern What you get when you do Perl++ twice. Doing it only once will curl your hair. You have to increment it eight times to shampoo your hair. Lather, rinse, iterate.
pipe A direct <B>connectionB> that carries the output of one <B>processB> to the input of another without an intermediate temporary file. Once the pipe is set up, the two processes in question can read and write as if they were talking to a normal file, with some caveats.
pipeline A series of <B>processesB> all in a row, linked by <B>pipesB>, where each passes its output stream to the next.
platform The entire hardware and software context in which a program runs. A program written in a platform-dependent language might break if you change any of the following: machine, operating system, libraries, compiler, or system configuration. The perl interpreter has to be <B>compiledB> differently for each platform because it is implemented in C, but programs written in the Perl language are largely platform independent.
pod The markup used to embed documentation into your Perl code. Pod stands for XPlain old documentationX. See Camel chapter 23, XPlain Old DocumentationX.
pod command A sequence, such as =head1, that denotes the start of a <B>podB> section.
pointer A <B>variableB> in a language like C that contains the exact memory location of some other item. Perl handles pointers internally so you donXt have to worry about them. Instead, you just use symbolic pointers in the form of <B>keysB> and <B>variableB> names, or <B>hard referencesB>, which arenXt pointers (but act like pointers and do in fact contain pointers).
polymorphism The notion that you can tell an <B>objectB> to do something generic, and the object will interpret the command in different ways depending on its type. [< Greek XXXX- + XXXXX, many forms.]
port The part of the address of a TCP or UDP socket that directs packets to the correct process after finding the right machine, something like the phone extension you give when you reach the company operator. Also the result of converting code to run on a different platform than originally intended, or the verb denoting this conversion.
portable Once upon a time, C code compilable under both BSD and SysV. In general, code that can be easily converted to run on another <B>platformB>, where XeasilyX can be defined however you like, and usually is. Anything may be considered portable if you try hard enough, such as a mobile home or London Bridge.
porter Someone who XcarriesX software from one <B>platformB> to another. Porting programs written in platform-dependent languages such as C can be difficult work, but porting programs like Perl is very much worth the agony.
possessive Said of quantifiers and groups in patterns that refuse to give up anything once theyXve gotten their mitts on it. Catchier and easier to say than the even more formal nonbacktrackable.
POSIX The Portable Operating System Interface specification.
postfix An <B>operatorB> that follows its <B>operandB>, as in $x++.
pp An internal shorthand for a Xpush- popX code; that is, C code implementing PerlXs stack machine.
pragma A standard module whose practical hints and suggestions are received (and possibly ignored) at compile time. Pragmas are named in all lowercase.
precedence The rules of conduct that, in the absence of other guidance, determine what should happen first. For example, in the absence of parentheses, you always do multiplication before addition.
prefix An <B>operatorB> that precedes its <B>operandB>, as in ++$x.
preprocessing What some helper <B>processB> did to transform the incoming data into a form more suitable for the current process. Often done with an incoming <B>pipeB>. See also <B>C preprocessorB>.
primary maintainer The author that PAUSE allows to assign <B>co-maintainerB> permissions to a <B>namespaceB>. A primary maintainer can give up this distinction by assigning it to another PAUSE author. See Camel chapter 19, XCPANX.
procedure A <B>subroutineB>.
process An instance of a running program. Under multitasking systems like Unix, two or more separate processes could be running the same program independently at the same timeXin fact, the fork function is designed to bring about this happy state of affairs. Under other operating systems, processes are sometimes called XthreadsX, XtasksX, or XjobsX, often with slight nuances in meaning.
program See <B>scriptB>.
program generator A system that algorithmically writes code for you in a high-level language. See also <B>code generatorB>.
progressive matching <B>Pattern matchingB> matching>that picks up where it left off before.
property See either <B>instance variableB> or <B>character propertyB>.
protocol In networking, an agreed-upon way of sending messages back and forth so that neither correspondent will get too confused.
prototype An optional part of a <B>subroutineB> declaration telling the Perl compiler how many and what flavor of arguments may be passed as <B>actual argumentsB>, so you can write subroutine calls that parse much like built-in functions. (Or donXt parse, as the case may be.)
pseudofunction A construct that sometimes looks like a function but really isnXt. Usually reserved for <B>lvalueB> modifiers like my, for <B>contextB> modifiers like scalar, and for the pick-your-own-quotes constructs, q//, qq//, qx//, qw//, qr//, m//, s///, y///, and tr///.
pseudohash Formerly, a reference to an array whose initial element happens to hold a reference to a hash. You used to be able to treat a pseudohash reference as either an array reference or a hash reference. Pseduohashes are no longer supported.
pseudoliteral An <B>operatorB> Xthat looks something like a literal, such as the output-grabbing operator, <literal moreinfo="none"‘>command`.
public domain Something not owned by anybody. Perl is copyrighted and is thus not in the public domainXitXs just <B>freely availableB> and <B>freely redistributableB>.
pumpkin A notional XbatonX handed around the Perl community indicating who is the lead integrator in some arena of development.
pumpking A <B>pumpkinB> holder, the person in charge of pumping the pump, or at least priming it. Must be willing to play the part of the Great Pumpkin now and then.
PV A Xpointer valueX, which is Perl Internals Talk for a char*.


qualified Possessing a complete name. The symbol $Ent::moot is qualified; $moot is unqualified. A fully qualified filename is specified from the top-level directory.
quantifier A component of a <B>regular expressionB> specifying how many times the foregoing <B>atomB> may occur.


race condition A race condition exists when the result of several interrelated events depends on the ordering of those events, but that order cannot be guaranteed due to nondeterministic timing effects. If two or more programs, or parts of the same program, try to go through the same series of events, one might interrupt the work of the other. This is a good way to find an <B>exploitB>.
readable With respect to files, one that has the proper permission bit set to let you access the file. With respect to computer programs, one thatXs written well enough that someone has a chance of figuring out what itXs trying to do.
reaping The last rites performed by a parent <B>processB> on behalf of a deceased child process so that it doesnXt remain a <B>zombieB>. See the wait and waitpid function calls.
record A set of related data values in a <B>fileB> or <B>streamB>, often associated with a unique <B>keyB> field. In Unix, often commensurate with a <B>lineB>, or a blank-lineXterminated set of lines (a XparagraphX). Each line of the /etc/passwd file is a record, keyed on login name, containing information about that user.
recursion The art of defining something (at least partly) in terms of itself, which is a naughty no-no in dictionaries but often works out okay in computer programs if youXre careful not to recurse forever (which is like an infinite loop with more spectacular failure modes).
reference Where you look to find a pointer to information somewhere else. (See <B>indirectionB>.) References come in two flavors: <B>symbolic referencesB> and <B>hard referencesB>.
referent Whatever a reference refers to, which may or may not have a name. Common types of referents include scalars, arrays, hashes, and subroutines.
regex See <B>regular expressionB>.
regular expression A single entity with various interpretations, like an elephant. To a computer scientist, itXs a grammar for a little language in which some strings are legal and others arenXt. To normal people, itXs a pattern you can use to find what youXre looking for when it varies from case to case. PerlXs regular expressions are far from regular in the theoretical sense, but in regular use they work quite well. HereXs a regular expression: /Oh s.*t./. This will match strings like XOh say can you see by the dawns early lightX and XOh sit!X. See Camel chapter 5, XPattern MatchingX.
regular expression modifier An option on a pattern or substitution, such as /i to render the pattern case- insensitive.
regular file A <B>fileB> thatXs not a <B>directoryB>, a <B>deviceB>, a named <B>pipeB> or <B>socketB>, or a <B>symbolic linkB>. Perl uses the Xf file test operator to identify regular files. Sometimes called a XplainX file.
relational operator An <B>operatorB> that says whether a particular ordering relationship is <B>trueB> about a pair of <B>operandsB>. Perl has both numeric and string relational operators. See <B>collating sequenceB>.
reserved words A word with a specific, built-in meaning to a <B>compilerB>, such as if or delete. In many languages (not Perl), itXs illegal to use reserved words to name anything else. (Which is why theyXre reserved, after all.) In Perl, you just canXt use them to name <B>labelsB> or <B>filehandlesB>. Also called XkeywordsX.
return value The <B>valueB> produced by a <B>subroutineB> or <B>expressionB> when evaluated. In Perl, a return value may be either a <B>listB> or a <B>scalarB>.
RFC Request For Comment, which despite the timid connotations is the name of a series of important standards documents.
right shift A <B>bit shiftB> that divides a number by some power of 2.
role A name for a concrete set of behaviors. A role is a way to add behavior to a class without inheritance.
root The superuser (UID == 0). Also the top-level directory of the filesystem.
RTFM What you are told when someone thinks you should Read The Fine Manual.
run phase Any time after Perl starts running your main program. See also <B>compile phaseB>. Run phase is mostly spent in <B>runtimeB> but may also be spent in <B>compile timeB> when require, do FILE, or eval STRING operators are executed, or when a substitution uses the /ee modifier.
runtime The time when Perl is actually doing what your code says to do, as opposed to the earlier period of time when it was trying to figure out whether what you said made any sense whatsoever, which is <B>compile timeB>.
runtime pattern A pattern that contains one or more variables to be interpolated before parsing the pattern as a <B>regular expressionB>, and that therefore cannot be analyzed at compile time, but must be reanalyzed each time the pattern match operator is evaluated. Runtime patterns are useful but expensive.
RV A recreational vehicle, not to be confused with vehicular recreation. RV also means an internal Reference Value of the type a <B>scalarB> can hold. See also <B>IVB> and <B>NVB> if youXre not confused yet.
rvalue A <B>valueB> that you might find on the right side of an <B>assignmentB>. See also <B>lvalueB>.


sandbox A walled off area thatXs not supposed to affect beyond its walls. You let kids play in the sandbox instead of running in the road. See Camel chapter 20, XSecurityX.
scalar A simple, singular value; a number, <B>stringB>, or <B>referenceB>.
scalar context The situation in which an <B>expressionB> is expected by its surroundings (the code calling it) to return a single <B>valueB> rather than a <B>listB> of values. See also <B>contextB> and <B>list contextB>. A scalar context sometimes imposes additional constraints on the return valueXsee <B>string contextB> and <B>numeric contextB>. Sometimes we talk about a <B>Boolean contextB> inside conditionals, but this imposes no additional constraints, since any scalar value, whether numeric or <B>stringB>, is already true or false.
scalar literal A number or quoted <B>stringB>Xan actual <B>valueB> in the text of your program, as opposed to a <B>variableB>.
scalar value A value that happens to be a <B>scalarB> as opposed to a <B>listB>.
scalar variable A <B>variableB> prefixed with $ that holds a single value.
scope From how far away you can see a variable, looking through one. Perl has two visibility mechanisms. It does <B>dynamic scopingB> of local <B>variablesB>, meaning that the rest of the <B>blockB>, and any <B>subroutinesB> that are called by the rest of the block, can see the variables that are local to the block. Perl does <B>lexical scopingB> of my variables, meaning that the rest of the block can see the variable, but other subroutines called by the block cannot see the variable.
scratchpad The area in which a particular invocation of a particular file or subroutine keeps some of its temporary values, including any lexically scoped variables.
script A text <B>fileB> that is a program intended to be <B>executedB> directly rather than <B>compiledB> to another form of file before <B>executionB>.

Also, in the context of <B>UnicodeB>, a writing system for a particular language or group of languages, such as Greek, Bengali, or Tengwar.

script kiddie A <B>crackerB> who is not a <B>hackerB> but knows just enough to run canned scripts. A <B>cargo-cultB> programmer.
sed A venerable Stream EDitor from which Perl derives some of its ideas.
semaphore A fancy kind of interlock that prevents multiple <B>threadsB> or <B>processesB> from using up the same resources simultaneously.
separator A <B>characterB> or <B>stringB> that keeps two surrounding strings from being confused with each other. The split function works on separators. Not to be confused with <B>delimitersB> or <B>terminatorsB>. The XorX in the previous sentence separated the two alternatives.
serialization Putting a fancy <B>data structureB> into linear order so that it can be stored as a <B>stringB> in a disk file or database, or sent through a <B>pipeB>. Also called marshalling.
server In networking, a <B>processB> that either advertises a <B>serviceB> or just hangs around at a known location and waits for <B>clientsB> who need service to get in touch with it.
service Something you do for someone else to make them happy, like giving them the time of day (or of their life). On some machines, well-known services are listed by the getservent function.
setgid Same as <B>setuidB>, only having to do with giving away <B>groupB> privileges.
setuid Said of a program that runs with the privileges of its <B>ownerB> rather than (as is usually the case) the privileges of whoever is running it. Also describes the bit in the mode word (<B>permission bitsB>) that controls the feature. This bit must be explicitly set by the owner to enable this feature, and the program must be carefully written not to give away more privileges than it ought to.
shared memory A piece of <B>memoryB> accessible by two different <B>processesB> who otherwise would not see each otherXs memory.
shebang Irish for the whole McGillicuddy. In Perl culture, a portmanteau of XsharpX and XbangX, meaning the #! sequence that tells the system where to find the interpreter.
shell A <B>commandB>-line <B>interpreterB>. The program that interactively gives you a prompt, accepts one or more <B>linesB> of input, and executes the programs you mentioned, feeding each of them their proper <B>argumentsB> and input data. Shells can also execute scripts containing such commands. Under Unix, typical shells include the Bourne shell (/bin/sh), the C shell (/bin/csh), and the Korn shell (/bin/ksh). Perl is not strictly a shell because itXs not interactive (although Perl programs can be interactive).
side effects Something extra that happens when you evaluate an <B>expressionB>. Nowadays it can refer to almost anything. For example, evaluating a simple assignment statement typically has the Xside effectX of assigning a value to a variable. (And you thought assigning the value was your primary intent in the first place!) Likewise, assigning a value to the special variable $| ($AUTOFLUSH) has the side effect of forcing a flush after every write or print on the currently selected filehandle.
sigil A glyph used in magic. Or, for Perl, the symbol in front of a variable name, such as $, @, and %.
signal A bolt out of the blue; that is, an event triggered by the <B>operating systemB>, probably when youXre least expecting it.
signal handler A <B>subroutineB> that, instead of being content to be called in the normal fashion, sits around waiting for a bolt out of the blue before it will deign to <B>executeB>. Under Perl, bolts out of the blue are called signals, and you send them with the kill built-in. See the %SIG hash in Camel chapter 25, XSpecial NamesX and the section XSignalsX in Camel chapter 15, XInterprocess CommunicationX.
single inheritance The features you got from your mother, if she told you that you donXt have a father. (See also <B>inheritanceB> and <B>multiple inheritanceB>.) In computer languages, the idea that <B>classesB> reproduce asexually so that a given class can only have one direct ancestor or <B>base classB>. Perl supplies no such restriction, though you may certainly program Perl that way if you like.
slice A selection of any number of <B>elementsB> from a <B>listB>, <B>arrayB>, or <B>hashB>.
slurp To read an entire <B>fileB> into a <B>stringB> in one operation.
socket An endpoint for network communication among multiple <B>processesB> that works much like a telephone or a post office box. The most important thing about a socket is its <B>network addressB> (like a phone number). Different kinds of sockets have different kinds of addressesXsome look like filenames, and some donXt.
soft reference See <B>symbolic referenceB>.
source filter A special kind of <B>moduleB> that does <B>preprocessingB> on your script just before it gets to the <B>tokenerB>.
stack A device you can put things on the top of, and later take them back off in the opposite order in which you put them on. See <B>LIFOB>.
standard Included in the official Perl distribution, as in a standard module, a standard tool, or a standard Perl <B>manpageB>.
standard error The default output <B>streamB> for nasty remarks that donXt belong in <B>standard outputB>. Represented within a Perl program by the output> <B>filehandleB> STDERR. You can use this stream explicitly, but the die and warn built-ins write to your standard error stream automatically (unless trapped or otherwise intercepted).
standard input The default input <B>streamB> for your program, which if possible shouldnXt care where its data is coming from. Represented within a Perl program by the <B>filehandleB> STDIN.
standard I/O A standard C library for doing <B>bufferedB> input and output to the <B>operating systemB>. (The XstandardX of standard I/O is at most marginally related to the XstandardX of standard input and output.) In general, Perl relies on whatever implementation of standard I/O a given operating system supplies, so the buffering characteristics of a Perl program on one machine may not exactly match those on another machine. Normally this only influences efficiency, not semantics. If your standard I/O package is doing block buffering and you want it to <B>flushB> the buffer more often, just set the $| variable to a true value.
Standard Library Everything that comes with the official perl distribution. Some vendor versions of perl change their distributions, leaving out some parts or including extras. See also <B>dual-livedB>.
standard output The default output <B>streamB> for your program, which if possible shouldnXt care where its data is going. Represented within a Perl program by the <B>filehandleB> STDOUT.
statement A <B>commandB> to the computer about what to do next, like a step in a recipe: XAdd marmalade to batter and mix until mixed.X A statement is distinguished from a <B>declarationB>, which doesnXt tell the computer to do anything, but just to learn something.
statement modifier A <B>conditionalB> or <B>loopB> that you put after the <B>statementB> instead of before, if you know what we mean.
static Varying slowly compared to something else. (Unfortunately, everything is relatively stable compared to something else, except for certain elementary particles, and weXre not so sure about them.) In computers, where things are supposed to vary rapidly, XstaticX has a derogatory connotation, indicating a slightly dysfunctional <B>variableB>, <B>subroutineB>, or <B>methodB>. In Perl culture, the word is politely avoided.

If youXre a C or C++ programmer, you might be looking for PerlXs state keyword.

static method No such thing. See <B>class methodB>.
static scoping No such thing. See <B>lexical scopingB>.
static variable No such thing. Just use a <B>lexical variableB> in a scope larger than your <B>subroutineB>, or declare it with state instead of with my.
stat structure A special internal spot in which Perl keeps the information about the last <B>fileB> on which you requested information.
status The <B>valueB> returned to the parent <B>processB> when one of its child processes dies. This value is placed in the special variable $?. Its upper eight <B>bitsB> are the exit status of the defunct process, and its lower eight bits identify the signal (if any) that the process died from. On Unix systems, this status value is the same as the status word returned by wait(2). See system in Camel chapter 27, XFunctionsX.
STDERR See <B>standard errorB>.
STDIN See <B>standard inputB>.
STDIO See <B>standard I/OB>.
STDOUT See <B>standard outputB>.
stream A flow of data into or out of a process as a steady sequence of bytes or characters, without the appearance of being broken up into packets. This is a kind of <B>interfaceB>Xthe underlying <B>implementationB> may well break your data up into separate packets for delivery, but this is hidden from you.
string A sequence of characters such as XHe said !@#*&%@#*?!X. A string does not have to be entirely printable.
string context The situation in which an expression is expected by its surroundings (the code calling it) to return a <B>stringB>. See also <B>contextB> and <B>numeric contextB>.
stringification The process of producing a <B>stringB> representation of an abstract object.
struct C keyword introducing a structure definition or name.
structure See <B>data structureB>.
subclass See <B>derived classB>.
subpattern A component of a <B>regular expressionB> pattern.
subroutine A named or otherwise accessible piece of program that can be invoked from elsewhere in the program in order to accomplish some subgoal of the program. A subroutine is often parameterized to accomplish different but related things depending on its input <B>argumentsB>. If the subroutine returns a meaningful <B>valueB>, it is also called a <B>functionB>.
subscript A <B>valueB> that indicates the position of a particular <B>arrayB> <B>elementB> in an array.
substitution Changing parts of a string via the s/// operator. (We avoid use of this term to mean <B>variable interpolationB>.)
substring A portion of a <B>stringB>, starting at a certain <B>characterB> position (<B>offsetB>) and proceeding for a certain number of characters.
superclass See <B>base classB>.
superuser The person whom the <B>operating systemB> will let do almost anything. Typically your system administrator or someone pretending to be your system administrator. On Unix systems, the <B>rootB> user. On Windows systems, usually the Administrator user.
SV Short for Xscalar valueX. But within the Perl interpreter, every <B>referentB> is treated as a member of a class derived from SV, in an object-oriented sort of way. Every <B>valueB> inside Perl is passed around as a C language SV* pointer. The SV <B>structB> knows its own Xreferent typeX, and the code is smart enough (we hope) not to try to call a <B>hashB> function on a <B>subroutineB>.
switch An option you give on a command line to influence the way your program works, usually introduced with a minus sign. The word is also used as a nickname for a <B>switch statementB>.
switch cluster The combination of multiple command- line switches (e.g., Xa Xb Xc) into one switch (e.g., Xabc). Any switch with an additional <B>argumentB> must be the last switch in a cluster.
switch statement A program technique that lets you evaluate an <B>expressionB> and then, based on the value of the expression, do a multiway branch to the appropriate piece of code for that value. Also called a Xcase structureX, named after the similar Pascal construct. Most switch statements in Perl are spelled given. See XThe given statementX in Camel chapter 4, XStatements and DeclarationsX.
symbol Generally, any <B>tokenB> or <B>metasymbolB>. Often used more specifically to mean the sort of name you might find in a <B>symbol tableB>.
symbolic debugger A program that lets you step through the <B>executionB> of your program, stopping or printing things out here and there to see whether anything has gone wrong, and, if so, what. The XsymbolicX part just means that you can talk to the debugger using the same symbols with which your program is written.
symbolic link An alternate filename that points to the real <B>filenameB>, which in turn points to the real <B>fileB>. Whenever the <B>operating systemB> is trying to parse a <B>pathnameB> containing a symbolic link, it merely substitutes the new name and continues parsing.
symbolic reference A variable whose value is the name of another variable or subroutine. By <B>dereferencingB> the first variable, you can get at the second one. Symbolic references are illegal under use strict "refs".
symbol table Where a <B>compilerB> remembers symbols. A program like Perl must somehow remember all the names of all the <B>variablesB>, <B>filehandlesB>, and <B>subroutinesB> youXve used. It does this by placing the names in a symbol table, which is implemented in Perl using a <B>hash tableB>. There is a separate symbol table for each <B>packageB> to give each package its own <B>namespaceB>.
synchronous Programming in which the orderly sequence of events can be determined; that is, when things happen one after the other, not at the same time.
syntactic sugar An alternative way of writing something more easily; a shortcut.
syntax From Greek XXXXXXXX, Xwith-arrangementX. How things (particularly symbols) are put together with each other.
syntax tree An internal representation of your program wherein lower-level <B>constructsB> dangle off the higher-level constructs enclosing them.
syscall A <B>functionB> call directly to the <B>operating systemB>. Many of the important subroutines and functions you use arenXt direct system calls, but are built up in one or more layers above the system call level. In general, Perl programmers donXt need to worry about the distinction. However, if you do happen to know which Perl functions are really syscalls, you can predict which of these will set the $! ($ERRNO) variable on failure. Unfortunately, beginning programmers often confusingly employ the term Xsystem callX to mean what happens when you call the Perl system function, which actually involves many syscalls. To avoid any confusion, we nearly always say XsyscallX for something you could call indirectly via PerlXs syscall function, and never for something you would call with PerlXs system function.


taint checks The special bookkeeping Perl does to track the flow of external data through your program and disallow their use in system commands.
tainted Said of data derived from the grubby hands of a user, and thus unsafe for a secure program to rely on. Perl does taint checks if you run a <B>setuidB> (or <B>setgidB>) program, or if you use the XT switch.
taint mode Running under the XT switch, marking all external data as suspect and refusing to use it with system commands. See Camel chapter 20, XSecurityX.
TCP Short for Transmission Control Protocol. A protocol wrapped around the Internet Protocol to make an unreliable packet transmission mechanism appear to the application program to be a reliable <B>streamB> of bytes. (Usually.)
term Short for a XterminalXXthat is, a leaf node of a <B>syntax treeB>. A thing that functions grammatically as an <B>operandB> for the operators in an expression.
terminator A <B>characterB> or <B>stringB> that marks the end of another string. The $/ variable contains the string that terminates a readline operation, which chomp deletes from the end. Not to be confused with <B>delimitersB> or <B>separatorsB>. The period at the end of this sentence is a terminator.
ternary An <B>operatorB> taking three <B>operandsB>. Sometimes pronounced <B>trinaryB>.
text A <B>stringB> or <B>fileB> containing primarily printable characters.
thread Like a forked process, but without <B>forkB>Xs inherent memory protection. A thread is lighter weight than a full process, in that a process could have multiple threads running around in it, all fighting over the same processXs memory space unless steps are taken to protect threads from one another.
tie The bond between a magical variable and its implementation class. See the tie function in Camel chapter 27, XFunctionsX and Camel chapter 14, XTied VariablesX.
titlecase The case used for capitals that are followed by lowercase characters instead of by more capitals. Sometimes called sentence case or headline case. English doesnXt use Unicode titlecase, but casing rules for English titles are more complicated than simply capitalizing each wordXs first character.
TMTOWTDI ThereXs More Than One Way To Do It, the Perl Motto. The notion that there can be more than one valid path to solving a programming problem in context. (This doesnXt mean that more ways are always better or that all possible paths are equally desirableXjust that there need not be One True Way.)
token A morpheme in a programming language, the smallest unit of text with semantic significance.
tokener A module that breaks a program text into a sequence of <B>tokensB> for later analysis by a parser.
tokenizing Splitting up a program text into <B>tokensB>. Also known as XlexingX, in which case you get XlexemesX instead of tokens.
toolbox approach The notion that, with a complete set of simple tools that work well together, you can build almost anything you want. Which is fine if youXre assembling a tricycle, but if youXre building a defranishizing comboflux regurgalator, you really want your own machine shop in which to build special tools. Perl is sort of a machine shop.
topic The thing youXre working on. Structures like while(<>), for, foreach, and given set the topic for you by assigning to $_, the default (topic) variable.
transliterate To turn one string representation into another by mapping each character of the source string to its corresponding character in the result string. Not to be confused with translation: for example, Greek XXXXXXXXXX transliterates into polychromos but translates into many-colored. See the tr/// operator in Camel chapter 5, XPattern MatchingX.
trigger An event that causes a <B>handlerB> to be run.
trinary Not a stellar system with three stars, but an <B>operatorB> taking three <B>operandsB>. Sometimes pronounced <B>ternaryB>.
troff A venerable typesetting language from which Perl derives the name of its $% variable and which is secretly used in the production of Camel books.
true Any scalar value that doesnXt evaluate to 0 or "".
truncating Emptying a file of existing contents, either automatically when opening a file for writing or explicitly via the truncate function.
type See <B>data typeB> and <B>classB>.
type casting Converting data from one type to another. C permits this. Perl does not need it. Nor want it.
typedef A type definition in the C and C++ languages.
typed lexical A <B>lexical variableB> lexical>that is declared with a <B>classB> type: my Pony $bill.
typeglob Use of a single identifier, prefixed with *. For example, *name stands for any or all of $name, @name, %name, &name, or just name. How you use it determines whether it is interpreted as all or only one of them. See XTypeglobs and FilehandlesX in Camel chapter 2, XBits and PiecesX.
typemap A description of how C types may be transformed to and from Perl types within an <B>extensionB> module written in <B>XSB>.


UDP User Datagram Protocol, the typical way to send <B>datagramsB> over the Internet.
UID A user ID. Often used in the context of <B>fileB> or <B>processB> ownership.
umask A mask of those <B>permission bitsB> that should be forced off when creating files or directories, in order to establish a policy of whom youXll ordinarily deny access to. See the umask function.
unary operator An operator with only one <B>operandB>, like ! or chdir. Unary operators are usually prefix operators; that is, they precede their operand. The ++ and XX operators can be either prefix or postfix. (Their position does change their meanings.)
Unicode A character set comprising all the major character sets of the world, more or less. See <>.
Unix A very large and constantly evolving language with several alternative and largely incompatible syntaxes, in which anyone can define anything any way they choose, and usually do. Speakers of this language think itXs easy to learn because itXs so easily twisted to oneXs own ends, but dialectical differences make tribal intercommunication nearly impossible, and travelers are often reduced to a pidgin-like subset of the language. To be universally understood, a Unix shell programmer must spend years of study in the art. Many have abandoned this discipline and now communicate via an Esperanto-like language called Perl.

In ancient times, Unix was also used to refer to some code that a couple of people at Bell Labs wrote to make use of a PDP-7 computer that wasnXt doing much of anything else at the time.

uppercase In Unicode, not just characters with the General Category of Uppercase Letter, but any character with the Uppercase property, including some Letter Numbers and Symbols. Not to be confused with <B>titlecaseB>.


value An actual piece of data, in contrast to all the variables, references, keys, indices, operators, and whatnot that you need to access the value.
variable A named storage location that can hold any of various kinds of <B>valueB>, as your program sees fit.
variable interpolation The <B>interpolationB> of a scalar or array variable into a string.
variadic Said of a <B>functionB> that happily receives an indeterminate number of <B>actual argumentsB>.
vector Mathematical jargon for a list of <B>scalar valuesB>.
virtual Providing the appearance of something without the reality, as in: virtual memory is not real memory. (See also <B>memoryB>.) The opposite of XvirtualX is XtransparentX, which means providing the reality of something without the appearance, as in: Perl handles the variable-length UTFX8 character encoding transparently.
void context A form of <B>scalar contextB> in which an <B>expressionB> is not expected to return any <B>valueB> at all and is evaluated for its <B>side effectsB> alone.
v-string A XversionX or XvectorX <B>stringB> specified with a v followed by a series of decimal integers in dot notation, for instance, v1.20.300.4000. Each number turns into a <B>characterB> with the specified ordinal value. (The v is optional when there are at least three integers.)


warning A message printed to the STDERR stream to the effect that something might be wrong but isnXt worth blowing up over. See warn in Camel chapter 27, XFunctionsX and the warnings pragma in Camel chapter 28, XPragmantic ModulesX.
watch expression An expression which, when its value changes, causes a breakpoint in the Perl debugger.
weak reference A reference that doesnXt get counted normally. When all the normal references to data disappear, the data disappears. These are useful for circular references that would never disappear otherwise.
whitespace A <B>characterB> that moves your cursor but doesnXt otherwise put anything on your screen. Typically refers to any of: space, tab, line feed, carriage return, or form feed. In Unicode, matches many other characters that Unicode considers whitespace, including the X-XX .
word In normal XcomputereseX, the piece of data of the size most efficiently handled by your computer, typically 32 bits or so, give or take a few powers of 2. In Perl culture, it more often refers to an alphanumeric <B>identifierB> (including underscores), or to a string of nonwhitespace <B>charactersB> bounded by whitespace or string boundaries.
working directory Your current <B>directoryB>, from which relative pathnames are interpreted by the <B>operating systemB>. The operating system knows your current directory because you told it with a chdir, or because you started out in the place where your parent <B>processB> was when you were born.
wrapper A program or subroutine that runs some other program or subroutine for you, modifying some of its input or output to better suit your purposes.
WYSIWYG What You See Is What You Get. Usually used when something that appears on the screen matches how it will eventually look, like PerlXs format declarations. Also used to mean the opposite of magic because everything works exactly as it appears, as in the three- argument form of open.


XS An extraordinarily exported, expeditiously excellent, expressly eXternal Subroutine, executed in existing C or C++ or in an exciting extension language called (exasperatingly) XS.
XSUB An external <B>subroutineB> defined in <B>XSB>.


yacc Yet Another Compiler Compiler. A parser generator without which Perl probably would not have existed. See the file perly.y in the Perl source distribution.


zero width A subpattern <B>assertionB> matching the <B>null stringB> between <B>charactersB>.
zombie A process that has died (exited) but whose parent has not yet received proper notification of its demise by virtue of having called wait or waitpid. If you fork, you must clean up after your child processes when they exit; otherwise, the process table will fill up and your system administrator will Not Be Happy with you.


Based on the Glossary of Programming Perl, Fourth Edition, by Tom Christiansen, brian d foy, Larry Wall, & Jon Orwant. Copyright (c) 2000, 1996, 1991, 2012 O’Reilly Media, Inc. This document may be distributed under the same terms as Perl itself.
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