Mercutio is talking about a medlar fruit, which was colloquially referred to as an "open arse," for reasons that can never be adequately explained. However, there is no such thing as a poperin pear -- it's another old-timey play on words. Separate "poperin" into its three syllables and you get an Elizabethan penis euphemism -- "pop 'er in."
Yep. Mercutio is saying, "What you need, my friend, is a chick who does anal."
Nicholas Joseph Crowley/Royal Shakespeare Company Collection
Mercutio, the original Dude-Bro.