Perl::Critic::Policy::Community::DeprecatedFeatures - Avoid features that have been deprecated or removed from Perl


While Perl::Critic::Policy::Community::StrictWarnings will expose usage of deprecated or removed features when a modern perl is used, this policy will detect such features in use regardless of perl version, to assist in keeping your code modern and forward-compatible.



Because the whitespace between an attribute list and assignment operator is not significant, it was possible to specify assignment to a variable with an empty attribute list with a construction like my $foo := 'bar'. This is deprecated in perl v5.12.0 to allow the possibility of a future := operator. Avoid the issue by either putting whitespace between the : and = characters or simply omitting the empty attribute list.


The magic "$[" in perlvar variable was used in very old perls to determine the index of the first element of arrays or the first character in substrings, and also allow modifying this value. It was discouraged from the start of Perl 5, its functionality changed in v5.10.0, deprecated in v5.12.0, re-implemented as in v5.16.0, and it is essentially a synonym for 0 under use v5.16 or no feature "array_base". While it is probably a bad idea in general, the modules Array::Base and String::Base can now be used to replace this functionality.


The \C regular expression character class would match a single byte of the internal representation of the string, which was dangerous because it violated the logical character abstraction of Perl strings, and substitutions using it could result in malformed UTF-8 sequences. It was deprecated in perl v5.20.0 and removed in perl v5.24.0. Instead, explicitly encode the string to UTF-8 using Encode to examine its UTF-8-encoded byte representation.


The ?PATTERN? regex match syntax is deprecated in perl v5.14.0 and removed in perl v5.22.0. Use m?PATTERN? instead.


An experimental feature was introduced in perl v5.14.0 to allow calling various builtin functions (which operate on arrays or hashes) on a reference, which would automatically dereference the operand. This led to ambiguity when passed objects that overload both array and hash dereferencing, and so was removed in perl v5.24.0. Instead, explicitly dereference the reference when calling these functions. The functions affected are each, keys, pop, push, shift, splice, unshift, and values.

Bare here-doc

Using << to initiate a here-doc would create it with an empty terminator, similar to <<'' , so the here-doc would terminate on the next empty line. Omitting the quoted empty string has been deprecated since perl 5, and is a fatal error in perl v5.28.0.


Passing an empty string or undef to chdir() would change to the home directory, but this usage is deprecated in perl v5.8.0 and throws an error in perl v5.24.0. Instead, call chdir() with no arguments for this behavior.

defined on array/hash

Using the function defined() on an array or hash probably does not do what you expected, and is deprecated in perl v5.6.2 and throws a fatal error in perl v5.22.0. To check if an array or hash is non-empty, test if it has elements.

 if (@foo) { ... }
 if (keys %bar) { ... }


This form of do to call a subroutine has been deprecated since perl 5, and is removed in perl v5.20.0.

NBSP in \N{...}

Use of the "no-break space" character in character names is deprecated in perl v5.22.0 and an error in perl v5.26.0.

POSIX character functions

Several character matching functions in are deprecated in perl v5.20.0. See the POSIX documentation for more details. Most uses of these functions can be replaced with appropriate regex matches.

 isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, issuper, isxdigit


The tmpnam() function from is deprecated in perl v5.22.0 and removed in perl v5.26.0. Use File::Temp instead.

qw(...) as parentheses

Literal parentheses are required for certain statements such as a for my $foo (...) { ... } construct. Using a qw(...) list literal without surrounding parentheses in this syntax is deprecated in perl v5.14.0 and a syntax error in perl v5.18.0. Wrap the literal in parentheses: for my $foo (qw(...)) { ... }.

require ::Foo::Bar

A bareword require (or use) starting with a double colon would inadvertently translate to a path starting with /. Starting in perl v5.26.0, this is a fatal error.


The method UNIVERSAL->import() and similarly passing import arguments to use UNIVERSAL is deprecated in perl v5.12.0 and throws a fatal error in perl v5.22.0. Calling use UNIVERSAL with no arguments is not an error, but serves no purpose.


This policy is part of Perl::Critic::Community.


This policy is not configurable except for the standard options.


This policy is incomplete, as many deprecations are difficult to test for statically. It is recommended to use perlbrew or perl-build to test your code under newer versions of Perl, with warnings enabled.


Dan Book,


Copyright 2015, Dan Book.

This library is free software; you may redistribute it and/or modify it under the terms of the Artistic License version 2.0.