I would start by using something other than a regex... Perhaps you could use a for loop instead:

my ($correct, $incorrect) = 0;
for(0 .. length($string)-1) {
    if( substr($string, $_, 1) eq substr($guess, $_, 1) ) {
        $correct ++;

    } else {
        $incorrect ++;
    }
}
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There's probably 600 ways to do it without regexes, but I think the only way you'll get a regex to work is with (?{ }) embedded code match thingies, and that's voodoo. I'm not saying it can't be done, but the documentation on them suggests you not use them.

Please see: perlre, for, substr.