I am going to assume you have a typo in your second array definition

Good point.
If the double "acct4" item in the original code is not a typo but rather intended to demonstrate special handling for duplicate entries, then the OP would need to explain how exactly such cases should be handled.

One possibility would be to strip all duplicate items from the list before merging (e.g. using the uniq function from List::MoreUtils). However, whether that is the correct thing to do would depend on the exact specification of the intended outcome in the general case.