From perlfunc's entry on split:

As a special case, specifying a PATTERN of space (' ') will split on white space just as split with no arguments does. Thus, split(' ') can be used to emulate awk's default behavior, whereas split(/ /) will give you as many null initial fields as there are leading spaces. A split on /\s+/ is like a split(' ') except that any leading whitespace produces a null first field. A split with no arguments really does a split(' ', $_) internally.