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};