I was part way through a long-winded explaination when the latest episode of Enterprise came on. In the meantime the other respondants have explained it much better than I was.

The only addition I would make is that the ++ serves only one purpose, that of inhibiting the "Useless use of hash slice in void context" warning message that results were it not there. I agree with Aristotle that assigning an empty list is a much clearer way of acheiving this, and is the way I will do this in future.

An added bonus is that for anything other than the smallest datasets, it is slightly more efficient.