Our friends over at MMORPGItalia had a chat with the development team of Perpetuum recently to talk about these details of the game. The concept of a persistent world is nothing new for MMOs, but this game seems to be bringing it to a new level -- don't get too confident if you're using a tree for cover in battle, for example. Your enemies have the ability to blow it away, leaving you exposed. However, you can create new cover if you dig quickly enough. This sort of interaction with the world can be found throughout the game, and changes you make to the world are permanent until someone else comes along and makes another change.
Perpetuum is a completely open, sandbox-style world in the truest sense: "anyone can go anywhere." While there are missions here and there, you're generally on your own. You interact with the world as a human who is remotely controlling robots, and where you take your robot is entirely up to you.
Check out the full story at MMORPGItalia for all the details on the persistent world of Perpetuum.