I have no precise definition of perimeter. Other perspectives on computer security use the term frequently, but without a crisp definition that I am aware of.
“Perimeter” suggests an area that is bounded by the perimeter. Indeed in computer security there is frequently a set of objects, or set of events among which signals are not security concerns, and neither are signals between objects or events outside the set. Security concerns itself with signals into or out of the set.
My note on kinds of information insulation refers to environments and I will use that term here. We will say that perimeters surround environments. The voluntary insulations, veiling and defending, have perimeter code within the environment that deals with the meaning of cross border signals so as to preserve the security or integrity of the environment. For the involuntary environments, confining and isolating, there is generally code outside the environment that attends to the same two issues. This is perimeter code in all cases.
This suggests the terms “open” and “closed” for involuntary and voluntary insulation respective in analogy to topological ideas wherein the boundary of a closed set is in the set but the boundary of an open set is outside. I doubt that this is useful. It does suggest a profitable duality however.
Conventional firewalls are mainly to limit the actions of external objects upon internal objects. Sometimes they have the opposite charter as well.