For Petruchio.
# tie %hash, 'Tie::Hash::Default', $def_value;

package Tie::Hash::Default;

use Tie::Hash;
@ISA = qw( Tie::StdHash );

my %def;

sub TIEHASH {
  my ($class, $default) = @_;
  my $self = $class->SUPER::TIEHASH;  # updated
  $def{$self} = $default;
  return $self;
}

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

1;

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In reply to Tie::Hash::Default by japhy

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