#!/usr/bin/perl -w

use strict;
use warnings;

use Data::Dumper;

my $ref = {
      a => { A => 1, B => 2, C => 3},
      b => { D => 1, E => 2, F => 4},
      c => { G => 2, H => 1, I => 2},
};

#print Dumper($ref);


my %hist;

foreach ($ref) {
    $hist{$_}++ for $_;
}

my @uni = keys %hist;

print "@uni\n";