The aspect of the perfect tense denotes action as having been already completed, in the writer’s mind, with the emphasis on the resulting state of affairs cf. BBG chapter 25 or this video. See the endings here. Typically, this is captured in English by using the helping verb “has”, “have” or “had”.
πεπιστευκα – I have believed
λελυκα – I have loosed
Perfects are easy to find because they reduplicate.