package Tie::Hash::Default;
require Tie::Hash;
our @ISA = qw/Tie::ExtraHash/;

sub TIEHASH {
    my ($class, $default) = @_;
    bless [ {}, $default ] => $class;
}

sub FETCH {
    my ($self, $key) = @_;
    exists $self->[0]->{$key}
     ? $self->[0]->{$key}
     : $self->[1];
}

#############
package main;

## -1 is the default value for nonexistant keys in %h

tie my %h, 'Tie::Hash::Default', -1;

$h{foo} = $h{bar} = 1;

print "$_ => $h{$_}\n"
    for qw/foo bar baz/;