#!/usr/bin/perl -w

use warnings; 
use strict;

BEGIN {
        my %SeenMerged = ();

        sub mergehashes{     # Dies on circular references
            my @hashrefs = @_;
            die "Passed a non hashref" if grep { ref $_ ne 'HASH' } @h
+ashrefs;
            my %merged = ();

            my @seen = grep { ref $_ eq 'HASH' }  @SeenMerged{@hashref
+s}; # Break circular links..

            if (@seen){
                die "contains a circular reference! bailing...";
            }

            @SeenMerged{@hashrefs} = @hashrefs;

            foreach my $h (@hashrefs){
                while (my ($k,$v) = each %$h ){
                    push @{$merged{$k}}, $v;
                }
            }

            while (my ($k,$v) = each %merged){
                my @hashes = grep { ref $_ eq 'HASH' } @$v;
                $merged{$k} = $v->[0] if (@$v == 1 && !ref $v->[0]);  
+  
                $merged{$k}  = mergehashes(@hashes) if @hashes;
            }

            delete @SeenMerged{@hashrefs};
            return \%merged;
        }

}

# EXAMPLE

use Data::Dumper;

my (%hash1,%hash2);
%hash1 = (
          red => 1,
          brown => {
                    green => 1,
                    blue => {
                             yellow => 1,
                            },
                    black => 1,
                   },
          gray => 1,
		  
         );

%hash2 = (
          white => 1,
          brown => {
                    purple => 1,
                   },
         );



my $merged = mergehashes(\%hash1, \%hash2 );

print Dumper($merged);


__END__
outputs

$VAR1 = {
          'gray' => 1,
          'white' => 1,
          'brown' => {
                       'blue' => {
                                   'yellow' => 1
                                 },
                       'purple' => 1,
                       'green' => 1,
                       'black' => 1
                     },
          'red' => 1
        };