The r of pari is changed to l before gha and aṅka,
As परिघः or पलिघः, पर्यङ्कः or पल्यङ्कः ॥ The word घ here means the word-form घ, and not the technical घ of तरप् and तमप् ॥ See 3.3.84 by which हन् is replaced by घ ॥
Vart:- So also, it must be stated, before the word योगः ॥ As, परियोगः or पलियोगः ॥,