As the thing you want to output is a hash ref, the statement print $h3 will output something like HASH(0x182f384). You will get something closer to your expected output by using Data::Dumper, swapping your print statement for use Data::Dumper; print Dumper $h3;. See How can I visualize my complex data structure? for more information.

The reason you are clobbering your initial hash is because your structure is a hash of hashes, so while they are different at the base level, you have the same object (a hash reference) associated with the key drug comparison. To make a shallow copy, I usually use Data::Dumper again, this time in an eval in a do: 

my $h1 = {
    'drug comparison' => 
    {
          '7003000.xml' => {
                             'entity' => 'a1, a2, a3'
                           },
          '70037559.xml' => {
                              'entity' => 'x1, x2, x3'
                            }
        }
};
my $copy = do{my $VAR1; eval Dumper $h1; $VAR1;};

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I would suggest making the copy in your union_of_h1_h2 sub.

In reply to Re: Generate union of the two hashes by kennethk
in thread Generate union of the two hashes by shan_emails

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