Something that could help perry, is stress coat api, it helps with the slime coat and such, at first I got it as a test thing, but I think it actually works and you can use it as a regular conditioner and it comes in decent quantity and price.
Also Indian almond leaves help calm down stress from water changes, as they do when being mailed in little bags.
Perhaps perry has indeed caught his fin somewhere, or perhaps and most likely he's just stress or impacted by the sudden water change. Keep an eye just in case you see anything odd.
For now just clean water will help him and time. Make sure to keep his enviroment as stress free as possible, perhaps stay away from the tank and give him some time to recover.
100% water changes are stressful by just a bit but as someone said if you do it gently and quickly it hardly has impactation on your fish. How I handle mine is I cup them with their old water while I do a quick rinsing of the tank, rid of the gunk and such. Then I let the cup float in the tank to adjust to the new temperature diff, then I add the fish straining the old water away and then quickly and gently leaving the fish in the tank with the net. Time is key with 100% water changes, the longer you take the higher the possibility of your fish getting stressed. (Depends on your fish's personality)
it is convienient to have water with conditioner sitting in a bottle or bucket for 24 hours to make sure all the chemicals are gone, if not an hour should be sufficient but the longer you have it with the conditioner the better. Then I let the heater heat up the water to the previous temp and then let the betta float with cup. Nothing impacts bettas more than a brusque change of temperature.
yes it is correct, 50% and 100%, twice a week water change, if you have a siphon use it to get rid of waste during the 50% if not a turkey baster is great for picking up uneaten food and poop.
hope this helps! keep us posted!