IMHO you are thinking about this problem incorrectly. If you are using an object framework, then you should probably be using an object if you need a complex data behavior. This means scores should be a subtype with fields that are the allowed values, perhaps like:

#!/usr/bin/perl

package Test::HashK;

use Moose;
use Moose::Util::TypeConstraints;

our @_allowed_colors = qw(red green blue);
enum 'colors', \@_allowed_colors;
has scores => ( is => 'rw', isa => 'Test::HashK::Score', default => su
+b{Test::HashK::Score->new()} ); # I want the allowed *keys* to be col
+ors

package Test::HashK::Score;

use Moose;
for my $color (@Test::HashK::_allowed_colors) {
    has $color =>
        is  => 'rw',
        isa => 'Num',
        ;
}
1;
[download]

If you do want to keep it as an explicit hash, rather than a blessed object, you could use Hash::Util's lock_keys or lock_ref_keys:

my @colors = qw(red green blue);
enum 'colors', \@colors;
has scores => 
  ( is => 'rw', 
    isa => 'HashRef', 
    default => sub {
        use Hash::Util 'lock_keys';
        my %hash; 
        lock_keys %hash, @colors;
        return \%hash;
      } 
  );
[download]

Of course, the user could easily bypass this by inserting their own value. As with most things in Perl, it gives you all the rope you want, and if you want to work outside provided frameworks, that's your issue.

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