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

my($hash);
$hash->{"AAA"}->{"KEY1"} = "VALUE1";
$hash->{"AAA"}->{"KEY2"} = "VALUE2";
$hash->{"AAA"}->{"KEY3"} = "VALUE3";
$hash->{"BBB"}->{"KEY1"} = "VALUEA";
$hash->{"BBB"}->{"KEY2"} = "VALUEB";
$hash->{"BBB"}->{"KEY3"} = "VALUEC";

for my $set (sort {$hash->{$a}->{"KEY2"} cmp $hash->{$b}->{"KEY2"}} (keys %{$hash})){
        print "$set:\n";
        for my $key (sort(keys %{$hash->{$set}})){
                print "\t$key: $hash->{$set}->{$key}\n";
        }
}

##</code><code>##

AAA:
        KEY1: VALUE1
        KEY2: VALUE2
        KEY3: VALUE3
BBB:
        KEY1: VALUEA
        KEY2: VALUEB
        KEY3: VALUEC