in reply to auto-magical flipped hashes

perlfaq4: How do I look up a hash element by value?:
Create a reverse hash: %by_value = reverse %by_key; $key = $by_value{$value}; That's not particularly efficient. It would be more space-efficient to use: while (($key, $value) = each %by_key) { $by_value{$value} = $key; } If your hash could have repeated values, the methods above will only find one of the associated keys. This may or may not worry you.
You could probably use tie to do this magically, using your FLIPPED key.
package SpecialHash; use Tie::Hash; @SpecialHash::ISA = qw/Tie::StdHash/; sub FETCH { $_[1] eq "FLIPPED" ? { reverse %{ $_[0] } } : $_[0]->{$_[1]}; } package main; tie my %hash, 'SpecialHash'; @hash{ qw/foo bar/ } = qw/baz quux/; use Data::Dumper; print Dumper $hash{FLIPPED};