sub possible_match {
    my ($word1, $word2) = @_;
    return undef unless length( $word1 ) == length( $word2 );
    my ( %one, %two );
    for ( 0 .. length( $word1 ) - 1) {
        my $c1 = substr($word1, $_, 1);
        my $c2 = substr($word2, $_, 1);
        return undef if exists $one{ $c1 } and $one{ $c1 } ne $c2;
        return undef if exists $two{ $c2 } and $two{ $c2 } ne $c1;
        $one{ $c1 } = $c2;
        $two{ $c2 } = $c1;
    }
    return \%one;
}