Hi All,

Thanks for all your answers.

now i used "Clone" module. so the address of the copying hash has been changed. so now the changes affected only in h3 hash.

use Clone qw(clone);

$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, $ref) = @_;
    my $output = clone($ai);
        
    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}->{$f
+ilename};
                }
            }
        }
    }
    return $output;
}
[download]

Wishes
Shanmugam A.