Using prints (as, for example print $a, $b, $c, $big, $equal;) just before line 20 would help you debug this.

And, free, at no extra charge (shipping and delivery, $19.97 or three easy payments of just $99.99 per month for four months), a couple hints:

1. Since you want to deal with numbers, use == rather than eq to test equality.
2. Format your code for readability -- chiefly by using indentation ... for example,
   

   if($first == $second) {                    # Indentation for readabili
   +ty
       $big = $first;
       $equal = $first;
   } elsif($first > $second) {                # but cuddled elses are mer
   +ely my preference
       $big=$first;
   } else {
       $big = $second;
   }
   [download]

3. Don't routinely use $a and $b as variables; they're special to sort. While they do no harm here, they would if you attacked this task by using sort.