an iterator provides much less functionality that an array, for instance, you can only access the elements sequentially and you can not get the number of elements in advance. So, usually, returning an array is better than an iterator.

Though, there are some cases where an iterator is preferred: when you don't want to let full access to the array or when you are using another representation internally to hold the values like a db table and don't want to make a Perl array holding all the values for (memory) performance reasons.

For instance, In my module Sort::Key::Merger that merges sorted lists, I used the iterator aproach so that the original lists and the result list were not required to fit in memory.