The perfect active is built on its own stem (the third principal part), has a short 3rd plural in -ere for -erunt, and often drops v in shortened forms: posuere = posuerunt, norim = noverim.
The perfect stem comes from the third principal part and may look quite unlike the present: pono, posui; differo, distuli; ago, egi; aufero, abstuli (abstulerit); nosco, novi. When a stem is unfamiliar, look up the present.
The 3rd plural -erunt has a short form -ere: posuere = posuerunt "they have placed". It looks like a present infinitive, but the stem is the perfect stem (posu-, not pon-).
Shortened (syncopated) forms drop -v- and often a vowel: norim = noverim, norunt = noverunt, nosse = novisse, amasti = amavisti, audiit = audivit, custodierit = custodiverit.
A few verbs have perfect forms with present meaning: novi "I know" (so norim "I know", subjunctive), memini "I remember" (meminisse, imperative memento), odi "I hate" (odisse "to hate").