There are at least two ways to ask this question.
1. Zach Wheeler is promoted to the majors, takes the mound once, pitches a perfect game, and then quits baseball to become a Trappist monk. What is his career WAR?
2. Take a perfectly league average starter. Add one extra start, in which he pitches a perfect game. How much does he add to his WAR?
Presumably the maximum WAR a single game performance should add is... 1.0. (Or perhaps 0.5.)