my %hash1 = ( foo => 'bar' );
my %hash2 = ( quz => 'baz' );
$hash1{somekey} = \%hash2;

use Data::Dumper;
print Dumper(\%hash1);
print Dumper( $hash1{somekey}{quz} );
__END__
$VAR1 = {
          'foo' => 'bar',
          'somekey' => {
                         'quz' => 'baz'
                       }
        };
$VAR1 = 'baz';

##</code><code>##

use Data::Dump;

my %hash1 = ( foo => 'bar' );
my %hash2 = ( quz => 'baz' );

# Case 1
%hash1 = (%hash1, %hash2);
# OR
$hash1{$_} = $hash2{$_} for keys %hash2;
dd \%hash1;  # { foo => "bar", quz => "baz" }

# Case 2
%hash1 = %hash2;
dd \%hash1;  # { quz => "baz" }