how could you adapt it to calculate second and third highest numbers?

something like:

use strict;

my @array = (1, 5, 4, 10, 20, 2, 1, 3, 7);
my @hi = (0) x 3;                # '0' assumes positive #'s

foreach (@array) {
   $_ <= $hi[0] and next;
   @hi = (sort $a<=>$b, @hi, $_)[-3..-1];
}

print "@hi";
[download]

(untested)   (and, as said, of course the sort method would be better, at least for small arrays, and would be even more better for finding the top three values:  (sort ..., @array)[-3..-1] )

  p