The exact meaning of perimeter can depend upon its context. For example:
"Stay inside the defensive perimeter." — the outer edges of the area that is defended
"It is common on the city's perimeter." — the outer edges of the city
"What is the perimeter of the square?" — the sum of the length of the sides of a geometric shape
As she spoke, great spotlights raked the compound, missing them by inches and seeking the outer perimeter of wire fence and mine fields and the woods beyond, where Hannah thought she saw shadows chasing shadows into the dark trees.
Chapter 17 (71% in)
There are no more uses of "perimeter" in The Devils Arithmetic.