When I've had to do this before and knew for sure that there was a one-to-one mapping between keys and values (i.e., no duplicate values), I've just created two hashes (or a single hash with two levels) and accessed them using subroutines or an object interface. Something like this:

my (%key_to_value, %value_to_key);

sub add_key_value {
    my ($key, $value) = @_;
    $key_to_value{$key} = $value;
    $value_to_key{$value} = $key;
}
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and so forth with more subroutines for deleting and changing mappings. Of course, this solution should use twice as much space as the original hash, and requires you to remember to use the subroutines whenever changing any of your data. You may be able to get around the second problem with a clever tied interface. Quick search of CPAN... In fact, it looks like Tie::Hash::TwoWay may do just this. Good luck!

-- Mike

--
just,my${.02}