It can also be done using a tied hash (with a speed penalty). I don't see an existing module on CPAN after a quick look, but it's it's easy to write your own. 
{
   package Hash::WithDefault;
   use Tie::Hash qw( );
   BEGIN { our @ISA = 'Tie::ExtraHash'; }

   use constant IDX_HASH    => 0;
   use constant IDX_DEFAULT => 1;
   use constant NEXT_IDX    => 2;

   sub FETCH {
      my ($self, $key) = @_;
      return ( exists( $self->[IDX_HASH]{$key} )
         ? $self->[IDX_HASH]{$key}
         : $self->[IDX_DEFAULT]
      );
   }
}


{
   tie my %hash, 'Hash::WithDefault', 'not in the alphabet';
   %hash = (
      a => 'a vowel',
      b => 'a consonant',
   );

   print("$_: $hash{$_}\n") for qw( a b ~ );
}

[download]

Output: 

a: a vowel
b: a consonant
~: not in the alphabet

[download]

In reply to Re: Default Hash Key by ikegami
in thread Default Hash Key by tford

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