Try something like this: 
sub score
{
    my ($x, $y) = @_;

    return $words_equal if $x eq $y;
    return 0 unless $x && $y;

    @{[$x =~ /[$y]/g]};
}

[download]
Basically, I'm treating $y as a character class to match on $x.

Now - there is a major assumption with this code - both words don't have characters twice. Thus, 'perl' and 'temp' is ok. But, 'perl' and 'etemp' isn't and neither is 'eperl' and 'temp'. Depending on which is first, the answer will be different. (The reason is that the characters in $y are treated once, but the characters in $x are treated as many times as it appears.)

I justify this cause you don't specify how to deal with multiple characters. If you are looking for just the number of unique characters they share, regardless of how many times it appears in either word, then do something like: 

sub score
{
    # Same as above, until:
    my $cnt1 = @{[$x =~ /[$y]/g]};
    my $cnt2 = @{[$y =~ /[$x]/g]};

    $cnt1 < $cnt2 ? $cnt1 : $cnt2;
}

[download]

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In reply to Re: Optimizing a string processing sub by dragonchild
in thread Optimizing a string processing sub by spurperl

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