Perl::Critic::Policy::ValuesAndExpressions::ProhibitInterpolationOfLiterals - Always use single quotes for literal strings.


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


Don't use double-quotes or qq// if your string doesn't require interpolation. This saves the interpreter a bit of work and it lets the reader know that you really did intend the string to be literal.

    print "foobar";     #not ok
    print 'foobar';     #ok
    print qq/foobar/;   #not ok
    print q/foobar/;    #ok

    print "$foobar";    #ok
    print "foobar\n";   #ok
    print qq/$foobar/;  #ok
    print qq/foobar\n/; #ok

    print qq{$foobar};  #preferred
    print qq{foobar\n}; #preferred

Use of double-quotes might be reasonable if the string contains single quote (') characters:

    print "it's me";    # ok, if configuration flag set


The types of quoting styles to exempt from this policy can be configured via the allow option. This must be a whitespace-delimited combination of some or all of the following styles: qq{}, qq(), qq[], and qq//.

This is useful because some folks have configured their editor to apply special syntax highlighting within certain styles of quotes. For example, you can tweak vim to use SQL highlighting for everything that appears within qq{} or qq[] quotes. But if those strings are literal, Perl::Critic will complain. To prevent this, put the following in your .perlcriticrc file:

    allow = qq{} qq[]

The flag allow_if_string_contains_single_quote permits double-quoted strings if the string contains a single quote (') character. It defaults to off; to turn it on put the following in your .perlcriticrc file:

    allow_if_string_contains_single_quote = 1




Jeffrey Ryan Thalhammer <>


Copyright (c) 2005-2021 Imaginative Software Systems. All rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of this license can be found in the LICENSE file included with this module.