I think I'm with you, (well done by the way), but I'm a little lost by the _: just after the split. Is the underscore functioning as a label at that point? I think so, but I've not seen that before.

Yes, it is a label. (If you don't figure it out when you see it, the redo should tip you off.) The underscore is considered to be a word character by most of Perl, so it can be used in places where most other punctuation cannot. For instance, a single underscore is also a valid subroutine name.