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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:


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.