sub score {
    my ($word1, $word2) = @_;
    my (%chars1, %chars2) = ();

    $chars1{$_}++ for split '', $word1;
    $chars2{$_}++ for split '', $word2;

    # the minimum of the two hashes is the number in common for each letter
    my $sum = 0;
    $sum += ($chars1{$_} < $chars2{$_}
               ? $chars1{$_}
               : $chars2{$_})   for keys %chars1;

    return $sum;
}

while (<DATA>) {
    chomp;
    print "$_: " . score(split /\s+/) . " in common\n";
}

__DATA__
perl monk
help temp
frood hoopy
bilbo baggins
jibber jaber