Pariguayo is a popular Dominican slang term for a 'chump,' 'fool,' or someone who is trying too hard to be something they're not. It's derived from the English phrase "party watcher," referring to someone who just stands around at a party instead of participating.
In the song, the artist dismisses his ex-lover's new boyfriend by calling him 'ese pariguayo' (that chump), implying the new guy is lame and no competition. It's a perfect example of colorful, regional slang that adds a layer of attitude to the lyrics.
In "Perfect," Lil Naay lays out his claim on a former lover who has moved on with someone new. He is completely convinced she still belongs to him, dismissing her new relationship by saying, "Que tú tienes a otro, pero eso no corre" (You have someone else, but that doesn't count). He reminds her of their connection, claiming, "Tú sabes que soy tu malo, mami, no te hagas" (You know I'm your bad boy, mami, don't pretend). His attitude is possessive and aggressive, even threatening her new partner with the line, "A mí me da lo mismo llenarlo de balas" (It's all the same to me to fill him with bullets).
He builds this whole persona of being the dangerous guy she secretly craves, talking about picking her up "con el peine 30" (with the 30-round magazine). He contrasts this with her current boyfriend, who he calls a "pariguayo" (loser). The song is incredibly direct and physical, with the central compliment being, "Ese culo está perfect / Botando lava" (That ass is perfect / Spitting lava). He's basically arguing that their intense, physical connection is something her new, safer relationship can't replace.