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Manual Reference Pages  -  ENCODING::WARNINGS (3)

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encoding::warnings - Warn on implicit encoding conversions



This document describes version 0.11 of encoding::warnings, released June 5, 2007.


    use encoding::warnings; # or FATAL to raise fatal exceptions

    utf8::encode($a = chr(20000));  # a byte-string (raw bytes)
    $b = chr(20000);                # a unicode-string (wide characters)

    # "Bytes implicitly upgraded into wide characters as iso-8859-1"
    $c = $a . $b;


    Overview of the problem

By default, there is a fundamental asymmetry in Perl’s unicode model: implicit upgrading from byte-strings to unicode-strings assumes that they were encoded in ISO 8859-1 (Latin-1), but unicode-strings are downgraded with UTF-8 encoding. This happens because the first 256 codepoints in Unicode happens to agree with Latin-1.

However, this silent upgrading can easily cause problems, if you happen to mix unicode strings with non-Latin1 data — i.e. byte-strings encoded in UTF-8 or other encodings. The error will not manifest until the combined string is written to output, at which time it would be impossible to see where did the silent upgrading occur.

    Detecting the problem

This module simplifies the process of diagnosing such problems. Just put this line on top of your main program:

    use encoding::warnings;

Afterwards, implicit upgrading of high-bit bytes will raise a warning. Ex.: Bytes implicitly upgraded into wide characters as iso-8859-1 at - line 7.

However, strings composed purely of ASCII code points (0x00..0x7F) will not trigger this warning.

You can also make the warnings fatal by importing this module as:

    use encoding::warnings FATAL;

    Solving the problem

Most of the time, this warning occurs when a byte-string is concatenated with a unicode-string. There are a number of ways to solve it:
o Upgrade both sides to unicode-strings

If your program does not need compatibility for Perl 5.6 and earlier, the recommended approach is to apply appropriate IO disciplines, so all data in your program become unicode-strings. See encoding, open and binmode in perlfunc for how.

o Downgrade both sides to byte-strings

The other way works too, especially if you are sure that all your data are under the same encoding, or if compatibility with older versions of Perl is desired.

You may downgrade strings with Encode::encode and utf8::encode. See Encode and utf8 for details.

o Specify the encoding for implicit byte-string upgrading

If you are confident that all byte-strings will be in a specific encoding like UTF-8, and need not support older versions of Perl, use the encoding pragma:

    use encoding utf8;

Similarly, this will silence warnings from this module, and preserve the default behaviour:

    use encoding iso-8859-1;

However, note that use encoding actually had three distinct effects:
o PerlIO layers for <B>STDINB> and <B>STDOUTB>

This is similar to what open pragma does.

o Literal conversions

This turns all literal string in your program into unicode-strings (equivalent to a use utf8), by decoding them using the specified encoding.

o Implicit upgrading for byte-strings

This will silence warnings from this module, as shown above.

Because literal conversions also work on empty strings, it may surprise some people:

    use encoding big5;

    my $byte_string = pack("C*", 0xA4, 0x40);
    print length $a;    # 2 here.
    $a .= "";           # concatenating with a unicode string...
    print length $a;    # 1 here!

In other words, do not use encoding unless you are certain that the program will not deal with any raw, 8-bit binary data at all.

However, the Filter => 1 flavor of use encoding will not affect implicit upgrading for byte-strings, and is thus incapable of silencing warnings from this module. See encoding for more details.


For Perl 5.9.4 or later, this module’s effect is lexical.

For Perl versions prior to 5.9.4, this module affects the whole script, instead of inside its lexical block.


perlunicode, perluniintro

open, utf8, encoding, Encode


Audrey Tang


Copyright 2004, 2005, 2006, 2007 by Audrey Tang <>.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

See <>

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perl v5.22.1 ENCODING::WARNINGS (3) 2015-10-17

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