The only exception I'm aware of is $r = () = 4..5, where the number of items is returned rather than the the last item. But by my definition above that would actually be an array, not a list, because you can assign to it.

But it ain't an array - you're assigning to a(n empty) sequence of scalars. And that $r ends up with the number of items has nothing to do with lists in scalar context. It's a feature of the assignment operator. Since () is an empty list, the second '=' is the list assignment operator. And the list assignment operator in scalar context returns the number of elements on its right hand side. (See "man perlop")