Very good catch, MarkM!. I didn't really thoroughly test my solution, leaving that as an exercise for the reader.

As for my solution, although this wasn't in the original specs, I also wanted to solve the problem without using some variant of split (which /(.)/g is). However, I'm blanking on it.

Although this may have been posted in the mainline discussion, I'll post this, as it Benchmark's faster than yours.

sub score
{
   my ($x, $y) = @_;
   return -1 if $x eq $y;

   my %x; $x{$_}++ for $x =~ /./g;
   my $score = 0;
   $x{$_} && $score++ for $y =~/./g;
   $score;
}
[download]

For 1 million iterations, this benches at 74.56sec. Yours benches at 122.51sec. :-)

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