#!/usr/bin/perl
use warnings; use strict;

my %seen;

for (0...1000) {

  my $a = [0..sprintf("%d", 1 + rand(20))];

  die "$_ iteratations: I've already seen $a"
    if exists $seen{$a};

  $seen{$a} = 1;
}

##</code><code>##

8 iteratations: I've already seen ARRAY(0x180bc0c) at at1.pl line 9.

##</code><code>##

$seen{$a} = $a; # capture the reference