That second one is a bit surprising, but when you look at perldoc -f sort:
sort SUBNAME LIST
sort BLOCK LIST
It sort of makes sense that 'sort get_stuff()' hits the first signature - it's sort, using the sorting rules in subname get_stuff, of an empty list.
It's not exactly DWIM. But we humans get a lot of advantage from understanding that subroutine name. It's moderately likely that you'll pass an empty list into a sort in code, and it would be highly annoying if your sort routine was sometimes interpreted as a source of values depending on if your list was populated. So, I think this is good behavior - you have to be very explicit for it not to be considered the sort rules function.