#!/usr/bin/env perl

use strict;
use warnings;
use List::Util qw(first);

my @array = (1,1,1,2,2,2,2,2,3,3,3,4,4,4,4,4,4,5,5,6);


my $idx = first {$array[$_] == 4} reverse 0..$#array;

print "$idx\n";