Here's a one-liner that capitalizes on citromatik's sort method above:

#!/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;
[download]

Prints:

$r{456}{d} ==> 7
$r{123}{a} ==> 3
[download]

UPDATE: Upon reading the question a second time, I decided that it would perhaps be more useful to maintain a hash that remembers the max valued second key for each first key, as below. The print line is just for show, since you can do anything with the hash once it is created.

my %rmax;
$rmax{$_} = (sort { $r{$_}->{$b} <=> $r{$_}->{$a} } keys %{$r{$_}})[0]
  for keys %r;
print "\$r{$_}{$rmax{$_}} ==> $r{$_}{$rmax{$_}}\n" for keys %rmax;
[download]

Prints:

$r{123}{a} ==> 3
$r{456}{d} ==> 7
[download]