%foo = (
  11 =>{ 4 => 'Four' },  
  33 =>{ 1 => 'One'  },
  2  =>{ 2 => 'Two'  }
);

my $ref_HoH = \%foo;

for my $k ( sort 
               { %{ $ref_HoH{$a} } <=> %{ $ref_HoH{$b} } }
               keys %$ref_HoH ) {
...
}

##</code><code>##

use strict;
use warnings;

my %foo = (
   11 => { '4'=>'Four', },
   33 => { '1'=>'One', },
   2 => { '2' => 'Two', },
);


my $ref_HoH = \%foo;

for my $k ( sort { $a <=> $b } keys %$ref_HoH ) {
   print "$k\n";
}

##</code><code>##

hf@flux[30] perl /tmp/testit
2
11
33