Sounds similar to my wound that had "tracks" branching off of it. If they don't wet the packing well before removing, the bandages may stick and it can be painful. Sometimes they are soaked with drainage, but I always made sure I had a bottle of sterile water near by. Malibu is correct in saying you should always take a pain pill about
a half hour before you're scheduled to have your dressings changed, it will take the edge off. I had wonderful nurses, they used to be so careful and hated it when they hurt me.
I eventually started using a wound vac, and it seemed that the sponges were a little easier to cut and shape and pack than the gauze although they still had to shove them in with the big, wooden end of the sterile Q-tip type things. I don't know if you could use a wound vac in the perianal area though a good nurse could probably figure out a way to do it, but they're supposed to really cut down on healing time.
I would imagine once the wound gets narrow enough you'll have to quit packing and hope it heals. My worry was that one of my tracks wouldn't heal the entire way and I'd have an abscess.