#!/usr/bin/perl
use strict;

my %r;
$r{123}{a}=3;
$r{123}{b}=2;
$r{456}{c}=5;
$r{456}{d}=7;

map { print "\$r{$_->[0]}{$_->[1]} ==> $r{$_->[0]}{$_->[1]}\n" }
map { [$_,(sort { $r{$_}->{$b} <=> $r{$_}->{$a} } keys %{$r{$_}})[0]]}
  keys %r;