"standard perturbation theory" - what exactly is meant?

hi,

could someone please help me out with the question in the title, in the following context:

the quantization around trivial classical solutions can be done via the minkowskian path integral, while instanton solutions arise in the euclidean formalism (using a semiclassical approximation, in the sense of stationary phase approximation and steepest descent).

however, i'm not sure what the author (Rajamaran) means with standard perturbation theory and the assertion that instanton solutions are non-perturbative classical solutions.

Thanks a lot.

(i hope my question makes sense, i'm quite new at qm)