1. The -1 and the 2 within square brackets inside the partition_usage sub. What is their exact purpose?

The construct before these square brackets return lists. [2] picks the third item of that list (remember, indexes are zero-based) (here: the third whitespace delimited field of the input), and [-1] returns the last list item.

2. If i had to glob for zipped logfiles within subdirectories how could i do it?

If you have fixed depth, say 2, you could write */*/*.log.gz. If not you'd have to use File::Find (or File::Find::Rule, which beginners seem to like better).