I could have sworn there was a CPAN module that did this. Tie::Hash::Default or something similarly-named. But I can't find it.

Anyway, this isn't too hard to implement with tie:

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/;
[download]

Be careful with this though. You have to make sure you initialize hash values that you are going to alter. A lot of magic/useful things depend on the default value being undef:

$h{asdf}++;        ## $h{asdf} == 0
$h{jkl} .= "jlk";  ## $h{jkl} eq "-1jkl"

use strict;
$h{xyz}{blah} = 1; ## Can't use string ("-1") as a HASH ref ..
[download]

... although using exists on the tied hash will still work as expected.