Perl::Critic::Policy::CodeLayout::ProhibitFatCommaNewline - keep a fat comma on the same line as its quoted word


This policy is part of the Perl::Critic::Pulp add-on. It reports a newline between a fat comma and preceding bareword for Perl builtins,

    my %h = (caller         # bad, builtin called as a function
             => 'abc');

And for all words when targeting Perl 5.6 and earlier,

    use 5.006;
    my %h = (foo            # bad, all words in perl 5.6 and earlier
             => 'def');

When there's a newline between the word and the fat comma like this the word executes as a function call (builtins always, and also user defined in Perl 5.6 and earlier), giving its return value rather than a word string.

Such a return value is probably not what was intended and on that basis this policy is under the "bugs" theme and medium severity (see "POLICY THEMES" in Perl::Critic).


Perl builtin functions with a newline always execute and give their return value rather than a the quoted word.

    my %h = (print          # bad, builtin print() executes
             => "abc");
    # %h is key "1" value "abc"

The builtin is called with no arguments and that might provoke a warning from some, but others like print will quietly run.

Dashed builtin names such as -print are also function calls, with a negate operator.

    my %h = (-print       # bad, print() call and negate
             => "123");
    # h is key "-1" value "123"

For the purposes of this policy the builtins are is_perl_builtin() from Perl::Critic::Utils. It's possible this is more builtins than the particular Perl in use, but guarding against all will help if going to a newer Perl in the future.


In Perl 5.6 and earlier all words foo execute as a function call when there's a newline before the fat comma.

    sub foo {
      return 123
    my %h = (foo
             => "def");
    # in Perl 5.6 and earlier %h is key "123" value "def"

Under use strict an error is thrown if no such function, in the usual way. A word builtin is a function call if it exists (with a warning about being interpreted that way), or a bareword if not.

This policy prohibits all words with newline before fat comma when targeting Perl 5.6 or earlier. This means either an explicit use 5.006 or smaller, or no such minimum use at all.

One subtle way an executing word with newline before fat comma can go undetected (in 5.6 and earlier still) is an accidental redefinition of a constant,

    use constant FOO => "blah";
    use constant FOO
      => "some value";
    # makes a constant subr called blah (in Perl 5.6) might reject some return values from FOO(), eg. a number, but a string like "blah" here quietly expands and creates a constant blah().

The difference between Perl 5.6 and later Perl is that in 5.6 the parser only looked as far as a newline for a possible quoting => fat comma. In Perl 5.8 and later for non-builtins the lookahead continues beyond any newlines and comments. For Perl builtins the behaviour is the same, in all versions the lookahead stops at the newline.

Avoiding Problems

Putting the fat comma on the same line as the word ensures it quotes in all cases.

    my %h = (-print =>    # ok, fat comma on same line quotes

If for layout purposes you do want a newline then the suggestion is to give a string or perhaps a parenthesized expression since that doesn't rely on the => fat comma quoting. A fat comma can still emphasize a key/value pair.

    my %h = ('print'      # ok, string

Alternately if instead a function call is really what's intended (builtin or otherwise) then parens can be used in the normal way to ensure it's a call (as per perltrap the rule being "if it looks like a function, it is a function").

    my %h = (foo()        # ok, function call


As always if you don't care about this then you can disable ProhibitFatCommaNewline from your .perlcriticrc in the usual way (see "CONFIGURATION" in Perl::Critic),



Perl::Critic::Pulp, Perl::Critic, perlop



Copyright 2011, 2013, 2014, 2015, 2016, 2017, 2019, 2021 Kevin Ryde

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