Benchmark: timing 10000 iterations of ZZamboni, chromatic...
  ZZamboni: 12 wallclock secs ( 5.64 usr +  0.00 sys =  5.64 CPU)
 chromatic: 19 wallclock secs ( 8.10 usr +  0.01 sys =  8.11 CPU)

##</code><code>##

#!/usr/bin/perl -w
   use strict;
   use Benchmark;
   my $tree = '';
   my %hash = (
           1 => 1,
           2 => { 3 => 4,
                   5 => {
                           6 => 7 },
                   },
           8 => 9,
           10 => {},
   );

sub ZZamboni {
  my %hash=%{$_[0]};
  my $level=$_[1];
  my $tree='';
  my $key;
  foreach $key (sort { $a <=> $b } keys %hash) {
     $tree .= "\t" x $level . "$key => ";
     if (ref($hash{$key})) {
        $tree .= "\n" . ZZamboni($hash{$key}, $level+1);
     }
     else {
        $tree .= $hash{$key} . "\n";
     }
  }
  return $tree;
}

sub chromatic {
   my %hash=%{$_[0]};
   my @stack;
   push @stack, \%hash;
   my $level = 0;
   my $tree='';

   while (@stack) {
           my ($hash_ref, $keys_ref);
           if (ref($stack[-1]) eq 'ARRAY') {
                   ($hash_ref, $keys_ref) = @{ pop @stack };
                   $level--;
           } else {
                   $hash_ref = pop @stack;
                   $keys_ref = [ sort { $b <=> $a } (keys %$hash_ref) ];
           }
           while (my $key = pop @{ $keys_ref }) {
                   my $value = $hash_ref->{$key};
                   if (ref($value) eq 'HASH') {
                           $tree .= "\t" x $level . "$key => \n";
                           push @stack, ([ $hash_ref, $keys_ref ], $value);
                           $level++;
                           last;
                   } else {
                           $tree .= "\t" x $level . "$key => $value\n";
   #                       print "on key $key, stack is ", $level, " levels deep.\n";
                   }
           }
   }
   return $tree;
}

timethese(10000, {
        'ZZamboni' => sub { ZZamboni(\%hash, 0) },
        'chromatic' => sub { chromatic(\%hash) },
        });