The questions are far too easy :)

1. $a is a global that can be used without declaring it because sorting would otherwise be a pain in the but.
2. += is a "don't care" operator. $a = $a + $_ would have generated a "use of uninitialized value in ..."-warning.
3. See 1 and 2
4. See 1 - you probably tried to avoid confusion, which is a good thing. Perhaps I should have done so to, but I could really not care less for a three-liner.

++ vs lbh qrpbqrq guvf hfvat n ge va Crey :)
Nabgure bar vs lbh qvq fb jvgubhg ernqvat n znahny svefg.
-- vs lbh hfrq OFQ pnrfne ;)
    - Whreq