
Certified GKH Gardening Expert
Experts say you should leave an inch of stem on the plant. I'm not seeing any research that suggest the remaining stem should be treated with anything. Unless you see signs of problems with the plant, it should dry up and be fine. On the other hand, if the remaining stem shows signs of rot, you may have other problems. Here are links to a couple of articles that should help:
https://www.gardeningknowhow.com/edible/fruits/pineapples/picking-pineapple-fruits.htm
https://www.gardeningknowhow.com/edible/fruits/pineapples/dealing-with-pineapple-problems.htm