Ok, let's start at the beginning. Why do you think Popcorn has a "hairball"? Is she not eating, not pooping, or something?
My first response to a not eating or not pooping situation is not to give pineapple. It is to assess for gas in the intestines and treat for that. Pineapple can be helpful in some situations but the enzymes present in it rarely survive the acidic stomach to reach the area in the intestines where hair may be impeding the movement of food through them.
The pineapple juice also has to be fresh-squeezed, not from a can or bottle.
It is my opinion that the reason pineapple juice seems to help some bunnies in GI stasis is that it is acidic, which is the correct pH for good GI bacteria to thrive, it is tasty so it encourages them to drink, and it has some sugar which can be helpful for a bunny that has not been eating for a long time due to stasis and has low blood sugar. USUALLY, sugar is bad in stasis--it does not promote the right GI bacteria. However, if a bunny has not been eating for 12+ hours, their blood sugar will be low, which will make them feel bad and not get the gut moving. Pineapple juice is a last resort in my opinion for stasis.
Mom to Tony, Muffin, Frida, and Big Bunny