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
She'd booked a CorpSeCorps man as protection in the taxi ride from the sealed bullet-train station through the short stretch of pleebland that had to be crossed before reaching the perimeter wall of the Module, which was the usual thing to do.
There are no more uses of "perimeter" in Oryx and Crake.