#!/usr/bin/perl -w

use Data::Dumper;

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

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

# merge ( \%hash1,  \%hash2 ) returns the merge
# If the two hashes have a common (possibly deep) key
# not pointing to hashes, it dies trying to force
# the reference to a hash reference
sub merge {
    my ( $h1, $h2 ) = @_;
    my %h3;
    for my $key ( keys %$h1 ) {
	if ( exists $h2->{$key} ) {
	    $h3{$key} = merge( $h1->{$key}, $h2->{$key} );
	} else {
	    $h3{$key} = $h1->{$key};
	}
    }
    for my $key ( keys %$h2 ) {
	unless ( exists $h1->{$key} ) {
	    $h3{$key} = $h2->{$key};
	}
    }
    return \%h3;
}

print Dumper ( merge( \%hash1, \%hash2 ) );