This policy is part of the "Perl::Critic::Pulp" add-on. It requires that when you use the hash style
multiple constants of "use constant" that you explicitly declare either Perl 5.8 or "constant" 1.03 or
higher.
use constant { AA => 1, BB => 2 }; # bad
use 5.008;
use constant { CC => 1, DD => 2 }; # ok
use constant 1.03;
use constant { EE => 1, FF => 2 }; # ok
use constant 1.03 { GG => 1, HH => 2 }; # ok
The idea is to keep you from using the multi-constant feature in code which might run on Perl 5.6, or
might in principle still run there. On that basis this policy is under the "compatibility" theme (see
"POLICY THEMES" in Perl::Critic).
If you declare "constant 1.03" then the code can still run on Perl 5.6 and perhaps earlier if the user
gets a suitably newer "constant" module from CPAN. Or of course for past compatibility just don't use
the hash style at all!
Details
A version declaration must be before the first multi-constant, so it's checked before the multi-constant
is attempted and gives an obscure error.
use constant { X => 1, Y => 2 }; # bad
use 5.008;
A "require" for the perl version is not enough since "use constant" is at "BEGIN" time, before plain
code.
require 5.008; # doesn't run early enough
use constant { X => 1, Y => 2 }; # bad
But a "require" within a "BEGIN" block is ok (a past style, still found occasionally).
BEGIN { require 5.008 }
use constant { X => 1, Y => 2 }; # ok
BEGIN {
require 5.008;
and_other_setups ...;
}
use constant { X => 1, Y => 2 }; # ok
Currently "ConstantPragmaHash" pays no attention to any conditionals within the "BEGIN", it assumes any
"require" there always runs. It could be tricked by some obscure tests but hopefully anything like that
is rare or does the right thing anyway.
A quoted version number like
use constant '1.03'; # no good
is no good, only a bare number is recognised by "use" and acted on by ConstantPragmaHash. A string like
that goes through to "constant" as if a name to define (which you'll see it objects to as soon as you try
run it).
Drawbacks
Explicitly adding required version numbers in the code can be irritating, especially if other things
you're doing only run on 5.8 up anyway. But declaring what code needs is accurate, it allows maybe for
backports of modules, and explicit versions can be grepped out to create or check Makefile.PL or Build.PL
prereqs.
As always if you don't care about this or if you only ever use Perl 5.8 anyway then you can disable
"ConstantPragmaHash" from your .perlcriticrc in the usual way (see "CONFIGURATION" in Perl::Critic),
[-Compatibility::ConstantPragmaHash]
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