#!/usr/bin/perl -w
 
my @string = (34, 56, 78, 90, 98, 76, 54, 32, 10, 12, 13, 16, 19, 20, 10, 56);
 
sub nth_iter{
  my ($item, $n, $list) = @_;
  ( grep  $list->[$_] == $item,  0..$#$list )[--$n] or -1;
}
 
print nth_iter(10, 3, \@string);