The Perl Toolchain Summit needs more sponsors. If your company depends on Perl, please support this very important event.


Perl::Critic::Policy::InputOutput::RequireEncodingWithUTF8Layer - Write open $fh, q{<:encoding(UTF-8)}, $filename; instead of open $fh, q{{<:utf8}, $filename;.


This Policy is part of the core Perl::Critic distribution.


Use of the :utf8 I/O layer (as opposed to :encoding(UTF8) or :encoding(UTF-8)) was suggested in the Perl documentation up to version 5.8.8. This may be OK for output, but on input :utf8 does not validate the input, leading to unexpected results.

An exploit based on this behavior of :utf8 is exhibited on PerlMonks at The exploit involves a string read from an external file and sanitized with m/^(\w+)$/, where $1 nonetheless ends up containing shell meta-characters.

To summarize:

 open $fh, '<:utf8', 'foo.txt';             # BAD
 open $fh, '<:encoding(UTF8)', 'foo.txt';   # GOOD
 open $fh, '<:encoding(UTF-8)', 'foo.txt';  # BETTER

See the Encode documentation for the difference between UTF8 and UTF-8. The short version is that UTF-8 implements the Unicode standard, and UTF8 is liberalized.

For consistency's sake, this policy checks files opened for output as well as input, For complete coverage it also checks binmode() calls, where the direction the operation can not be determined.


This Policy is not configurable except for the standard options.


Because Perl::Critic does a static analysis, this policy can not detect cases like

 my $encoding = ':utf8';
 binmode $fh, $encoding;

where the encoding is computed.




perldoc -f binmode


Thomas R. Wyant, III wyant at cpan dot org


Copyright (c) 2010-2011 Thomas R. Wyant, III

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