If you were to sit and eat like a pound or more of raw, fresh pineapple it absolutely would give you chemical burns unless you took precautions like washing your mouth out with water frequently. The bromelain in it breaks down the protective lining in your mouth and the acidity of the juice starts dissolving the now-unprotected flesh, but you really have to just be completely soaked in it for a while for that to happen.

Smaller quantities and pineapple that's cooked, mixed into other foods, or that's even just been sitting in the fridge for a few days doesn't have the same effect.