As shown by its data dumper output, $data_b is a reference to an array. The referenced array has a single element: a reference to a hash with four elements as shown.

Assuming a successful operation to merge the contents of the hash referenced by $data_a, what would you then want the data dumper output of $data_b to look like?

Update: The Perl Data Structures Cookbook (perldsc) may prove helpful now and in the future.