Some experimentation:

#!/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;
}
[download]

This dies unpredictably (varying number of iterations and at different addresses), eg:

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

However, if I change the last line to

$seen{$a} = $a; # capture the reference
[download]

..then my program does run to completion.

Addresses are unique when arrays are current. But they may become invalid and/be reused when arrays go out of scope.