One simple way of doing this is by sorting (by value) each sub-hash:

use strict;
use warnings;

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

for my $href (keys %r){
  my $maxkey = (sort { $r{$href}->{$b} <=> $r{$href}->{$a} } keys %{$r
+{$href}})[0];

  print "\$r{$href}{$maxkey} ==> $r{$href}{$maxkey}\n";
}
[download]

Outputs the desired result:

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

Hope this helps

citromatik