Perturbation from the macroscopic point of view, it means some disturbances that happen in a small time scale. From the point of Quantum Mechanics, often we retain the eigen states, then we will add another term/s which due to the perturbation. Think of another way, how about microscopic view, let's say atom? Of course in this case, it turns up to be interaction and the time scale is long enough that makes it uncomfortable.
What I want to bring up is Life can be a perturbation or interaction. Sometimes you feel that you have a hard time which makes you lost. You do not know what to do and you do not have any sense of direction and goal. You can call it as interaction as you do not know when the hard time will end. But after the time scale is long enough, then you will treat it as a kind of perturbation in your life when you have difference experiences in one life. (It remains me of Born Series or Dyson Series.)
Shit! What am I trying to express? My idea is simple: It is very hard for you to step up the first step, but once you have overcame the first step, then for whatever you are doing, you find that it is very easy.