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encoding::warnings(3pm)Perl Programmers Reference Guideencoding::warnings(3pm)


       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

           o   PerlIO layers for STDIN and STDOUT

               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


       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(1), perluniintro(1)

       open(3), utf8(3), encoding(3), Encode(3)


       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 <>

perl v5.24.0                      2016-03-01           encoding::warnings(3pm)

perl 5.24 - Generated Sun Nov 6 18:13:12 CST 2016
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