The Perfect is opposed to the Present, and has -o or lengthened root vowel; it is often reduplicated, generally with vowel -e; and it has special endings, sg. -a, -tha, -e; third pl. -r. Not all of these conditions have to be met by every Perfect stem, but those which do not comply with them (but for reduplication) are not regular.
NOTE. The original Perfect does not have a Tense or Voice value, it is only when opposed to the Pluperfect (Past Perfect) that it is Present. It is probably an older stative, which eventually became the state derived from the action.
I. The Root vowel is usually -o/-o; as, gigno /gegona (gegn-me), know; bhindho / bhondh (bhndh-me), bind; bheudho / bhoudh (bhdh-me), offer (cf. bid); kano / kekona, sing; etc.
There are also (dialectal) Perfects with long Root vowel; as, lat. sedeo / sede, sit; edo / ede, eat; cemio / ceme, come; ago / age, act; ger. sleb / sesleb, sleep; etc.
NOTE. After the laryngeals' theory, for some scholars, these lengthened vowels come from an older (wether IE II or PIE) vowel plus *-h.
II. The root vowel in the Perfect reduplication is generally e, but not always; Perfect reduplications in -i and -u are also possible.
NOTE. The Southern Dialects made reduplication obligatory, but the Northern didn't, what obviously means that it is not systematic in this Europaio. Verbs which are not reduplicated in the Perfect are not irregulars, but they have always reduplication when the Present is reduplicated, as in bhibher / bhebher.
III. The Endings of the Perfect are -a, -tha, -e, for the singular, and -me, -te, -(e)r for the plural.