There are numerous modules (such as the ones already described), or Storable’s dcopy function, which can help you here.

The essential problem here is that the easiest way to “copy a data structure” (or, that is to say, to think that you have done so...) involves the use of “references,” which are rather like “pointers” in other, more primitive languages.   Doing a deep copy, using any of the tools described, guarantees that you are left with a new data structure.