You forgot about the match operator:

sub match {
    my $sum = 0;
    for my $l ('A'..'Z') {
        my $acount = 0;
        ++$acount while($aa =~ /$l/g);
        my $bcount = 0;
        ++$bcount while($bb =~ /$l/g);
        $sum += abs($acount-$bcount);
    }
    return $sum;
}
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