Q1) In this case:

• $i is initially assigned the length of the list (a list in scalar context returns the length).
• the loop runs while --$i (the "guard") is true. Basically it runs through the array backwards.

Q2)

• It looks a bit like a 2-D array, but it's actually an array slice. It's swapping the contents of two array elements. A 2-D array would have a $ rather than a @ at the front.