Question

Let’s say I have a hashref constant like the following:

use constant DOGS => {
    Lassie => 'collie',
    Benji  => 'mutt',
    Scooby => 'great dane',
    Goofy  => '???'
};

How can I dereference it properly to get say.. they keys out of it?

warn ref DOGS; # HASH at untitled line 12.
warn keys( %{DOGS} ); # Warning: something's wrong (empty list)
warn keys( DOGS ); # Type of arg 1 to keys must be hash (not constant item)

The following is the only way I can seem to make it work:

my $dogs = DOGS;
warn keys( %$dogs ); # LassieBenjiGoofyScooby at untitled line 15.

What am I doing wrong?

Answer 1

This will usually work for you:

%{DOG()}

Constants are generally just subs. But for ease (and looks), you might prefer to use Readonly, as suggested in PBP.

Readonly::Hash my %DOG => 
    ( Lassie => 'collie'
    , Benji  => 'mutt'
    , Scooby => 'great dane'
    , Goofy  => '???'
    );