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";
}