This is the difference between a so-called “shallow copy” and a “deep” one.   Perl has the concept of references, which are scalar-things which “refer to” other things.   (This is, by the way, how Perl appears to have “multi-dimensional,” even arbitrary, data-structures given that it actually only has scalars, lists, arrays, and hashes.)   It is easy to “copy” something such that what you’re actually doing is creating a structure of references to the original thing ... easy, memory-efficient, and it usually works great.   This is a “shallow” copy.