#!/usr/bin/env perl
use strict;
use warnings;
use Data::Dumper;

my %hash = (
   a => [ '1','2', ],
   b => [ '2','3', ],
   c => [ '1','2', ],
);

my $testing_now = 'a';
for (keys %hash) {
   next if ($testing_now eq $_);
   if ($hash{$testing_now} ~~ $hash{$_}) {
      print "$testing_now is duplicate of $_\n";
   }
   #This is an update! Forgot this next line in initial post
   $testing_now = $_
}

# Returns
a is duplicate of c
c is duplicate of a (.02 version)