The if module is used to conditionally load or unload another module.
use if CONDITION, MODULE => ARGUMENTS;
will load MODULE only if CONDITION evaluates to true.
The above statement has no effect unless CONDITION is true.
If the CONDITION does evaluate to true, then the above line has
the same effect as:
use MODULE ARGUMENTS;
The use of => above provides necessary quoting of MODULE.
If you dont use the fat comma (eg you dont have any ARGUMENTS),
then youll need to quote the MODULE.
The following line is taken from the testsuite for File::Map:
use if $^O ne MSWin32, POSIX => qw/setlocale LC_ALL/;
If run on any operating system other than Windows,
this will import the functions setlocale and LC_ALL from POSIX.
On Windows it does nothing.
The following is used to deprecate core modules beyond a certain version of Perl:
use if $] > 5.016, deprecate;
This line is taken from Text::Soundex 3.04,
and marks it as deprecated beyond Perl 5.16.
If you use Text::Soundex in Perl 5.18, for example,
and you have used warnings,
then youll get a warning message
(the deprecate module looks to see whether the
calling module was used from a core library directory,
and if so, generates a warning),
unless youve installed a more recent version of Text::Soundex from CPAN.
You can also specify to NOT use something:
no if $] ge 5.021_006, warnings => "locale";
This warning category was added in the specified Perl version (a development
release). Without the if, trying to use it in an earlier release would
generate an unknown warning category error.
Module::Requires can be used to conditionally load one or modules,
with constraints based on the version of the module.
Unlike if though, Module::Requires is not a core module.
Module::Load::Conditional provides a number of functions you can use to
query what modules are available, and then load one or more of them at runtime.
provide can be used to select one of several possible modules to load,
based on what version of Perl is running.