A slice of an empty list is still an empty list. Thus:
    @a = ()[1,0];           # @a has no elements
    @b = (@a)[0,1];         # @b has no elements

But:
    @a = (1)[1,0];          # @a has two elements
    @b = (1,undef)[1,0,2];  # @b has three elements

More generally, a slice yields the empty list if it indexes only beyond the end of a list:
    @a = (1)[  1,2];        # @a has no elements
    @b = (1)[0,1,2];        # @b has three elements