$h1 = {
	'drug comparison' => 
	{
          '7003000.xml' => {
                             'entity' => 'a1, a2, a3'
                           },
          '70037559.xml' => {
                              'entity' => 'x1, x2, x3'
                            }
        }
};

$h2 = {
	'drug comparison' => 
	{
          '7004562.xml' => {
                             'entity' => 'z1, z2, z3'
                           },
          '70037559.xml' => {
                              'entity' => 'e1, e2, e3'
                            }
        }
};
$h3 = union_of_h1_h2 ($h1, $h2);
print $h3;


sub union_of_h1_h2 {
	my ($ai, $reference) = @_;
	my %output = %$ai;
	my %ref = %$reference;
		
	foreach my $_ref (keys %ref) {
		unless (exists $output{$_ref}) {
			$output{$_ref} = $ref{$_ref};
		}else {
			foreach my $filename (keys %{$ref{$_ref}}) {
				if (!exists $output{$_ref}{$filename}) {
					$output{$_ref}{$filename} = $ref{$_ref}{$filename};
				}
			}
		}
	}
	return \%output;
}


##</code><code>##


$h3 = {
	'drug comparison' => 
	{
          '7003000.xml' => {
                             'entity' => 'a1, a2, a3'
                           },
          '70037559.xml' => {
                              'entity' => 'x1, x2, x3'
                            },
          '7004562.xml' => {
                             'entity' => 'z1, z2, z3'
                           }
        }
};