Here is a way to do this. This is more C than perl. I'd love for a monk to translate it. This defines a function that takes: the item to find, the teration to locate, an arry to search This returns the array index of the nth element. You get a -1 if none found.

@string = qw(34, 56, 78, 90, 98, 76, 54, 32, 10, 12, 13, 16, 19, 20, 1
+0, 56);

sub nth_iter {
    my($item, $n, @list) = @_;
    $i = -1;
    $match = 0;
    $len = @list;
    while ($i++ < $len) {
        if($list[$i] == $item) {
            $match++;
            if ($match == $n) {
                last;
            }
        }
    }
    if ($match != $n) {
        $i = -1;
    }
    return $i;
}
        
print nth_iter(78, 1, @string);
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Rock on! -Ty