Recursion is expensive, but sometimes it's the quickest solution. Never one to walk away from a programming challenge (that I can actually envision), here's what you have to do to get at all of the keys in a Hash of Hashes of indeterminate nesting.

The sort is optional, as is what you do with the keys and values, once you find them. Making this into a subroutine and changing the output appropriately is left as an exercise for the reader.

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

my @stack;
push @stack, \%hash;
my $tree = '';
my $level = 0;

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"
+;
        }
    }
}
print $tree;

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In reply to Iterate over HoH without Recursion by chromatic

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