If at all possible, its best to avoid either way: make the caller provide an array ref every time. That allows $date to be a blessed object whose underlying type is a hash but which has overloaded @{} array-dereference.

No matter how you code the decision, there is some ambiguity. The caller could be passing an array-based object yet intending that the split be done using the object's overloaded stringify, for instance.

In a sense, if you are checking UNIVERSAL::isa and then treating it as a plain array ref, you are voilating the object's encapsulation. Why should the underlying type matter to you? Conversely, it is often convenient for an object method to be able to pass the object if it were just an array ref.

To sum it all up, you are guessing the caller's intent, and anything that involves a guess should preferably be avoided.