Help: how to perfect roll

Hello everyone, i’m here because i really like this game a lot, i’m playing for about a week or so and it is really cool.
I’ m level 99 and past floor 100 so i’m trying to find/craft the perfect gear for my warrior. I read all topics for enchenting and rerolling but i didn’t quite understand the perfect rolling.

  1. How can i know, when i reroll, that i’m doing a perfect reroll?
  2. It affects only the affixes or increase/decrease the stats of the item (damage/armor value)?
  3. And what’s the matter of rerolling an ilvl 100 item?

Anyway thanks a lot for the game develloppers and keep up with your amazing work.