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.IX Title "Perl::Critic::Policy::InputOutput::RequireEncodingWithUTF8Layer 3pm"
.TH Perl::Critic::Policy::InputOutput::RequireEncodingWithUTF8Layer 3pm "2020-05-17" "perl v5.30.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
Perl::Critic::Policy::InputOutput::RequireEncodingWithUTF8Layer \- Write "open $fh, q{<:encoding(UTF\-8)}, $filename;" instead of "open $fh, q{<:utf8}, $filename;".
.SH "AFFILIATION"
.IX Header "AFFILIATION"
This Policy is part of the core Perl::Critic
distribution.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Use of the \f(CW\*(C`:utf8\*(C'\fR I/O layer (as opposed to \f(CW\*(C`:encoding(UTF8)\*(C'\fR or
\&\f(CW\*(C`:encoding(UTF\-8)\*(C'\fR) was suggested in the Perl documentation up to
version 5.8.8. This may be \s-1OK\s0 for output, but on input \f(CW\*(C`:utf8\*(C'\fR does not
validate the input, leading to unexpected results.
.PP
An exploit based on this behavior of \f(CW\*(C`:utf8\*(C'\fR is exhibited on PerlMonks
at <http://www.perlmonks.org/?node_id=644786>. The exploit involves a
string read from an external file and sanitized with \f(CW\*(C`m/^(\ew+)$/\*(C'\fR,
where \f(CW$1\fR nonetheless ends up containing shell meta-characters.
.PP
To summarize:
.PP
.Vb 3
\& open $fh, \*(Aq<:utf8\*(Aq, \*(Aqfoo.txt\*(Aq;             # BAD
\& open $fh, \*(Aq<:encoding(UTF8)\*(Aq, \*(Aqfoo.txt\*(Aq;   # GOOD
\& open $fh, \*(Aq<:encoding(UTF\-8)\*(Aq, \*(Aqfoo.txt\*(Aq;  # BETTER
.Ve
.PP
See the Encode documentation for the difference between
\&\f(CW\*(C`UTF8\*(C'\fR and \f(CW\*(C`UTF\-8\*(C'\fR. The short version is that \f(CW\*(C`UTF\-8\*(C'\fR implements the
Unicode standard, and \f(CW\*(C`UTF8\*(C'\fR is liberalized.
.PP
For consistency's sake, this policy checks files opened for output as
well as input. For complete coverage it also checks \f(CW\*(C`binmode()\*(C'\fR calls,
where the direction of operation can not be determined.
.SH "CONFIGURATION"
.IX Header "CONFIGURATION"
This Policy is not configurable except for the standard options.
.SH "NOTES"
.IX Header "NOTES"
Because \f(CW\*(C`Perl::Critic\*(C'\fR does a static analysis, this policy can not
detect cases like
.PP
.Vb 2
\& my $encoding = \*(Aq:utf8\*(Aq;
\& binmode $fh, $encoding;
.Ve
.PP
where the encoding is computed.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
PerlIO
.PP
Encode
.PP
\&\f(CW\*(C`perldoc \-f binmode\*(C'\fR
.PP
<http://www.socialtext.net/perl5/index.cgi?the_utf8_perlio_layer>
.PP
<http://www.perlmonks.org/?node_id=644786>
.SH "AUTHOR"
.IX Header "AUTHOR"
Thomas R. Wyant, \s-1III\s0 \fIwyant at cpan dot org\fR
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 2010\-2011 Thomas R. Wyant, \s-1III\s0
.PP
This program is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.