Setting bounds

I’m quite new to OpenGL and I’m working on an Asteriods clone.
I’d like let the ship appear on the other side of the map when it reaches on of the window’s bounds. I’m using perspective projection, and therefore I don’t know how to set the bounds of a window. I’d be very grateful if somebody told me a solution to my (simple) problem.
P.S.: I don’t want to use orthongal projection mode even though it would be simpler to set the bounds then.