element @a   @b
 0       1   aa
 1       2   aa
 2       1   ab
 3       1   ac
 4       2   ac
 5       2   ad
 6       1   ac
 7       2   ac

##</code><code>##

$found = 0;

foreach $j (1..@a-1) {
  if ($a eq $a[$j] && $b eq $b[$j]) {
    $found = 1;
    last;
  }
}

unless ($found) {do something}