I have no issue with brining back Perry at this point since he essentially costs nothing and we have few healthy WRs.
But, the reason Perry was cut is because he was never active and he was never active because he could not learn the playbook. If you can't learn the playbook you aren't getting on the field. But, maybe he has learned a lesson about working hard and he might know enough to at least play a few snaps per game.
As far as Perry overall, I looked at his play from last year and he just isn't a good route runner. His routes are sloppy and he doesn't seem to put forth full effort. His best skill is that he is 6' 6" and appears to have a knacks for finding soft spots in zones once the play has broken down. That's not really how this offense is run, but I suppose with Rattler at QB he will extend some plays and Perry might be useful as a 3rd WR in the 2 minute drill if the other team is playing zone.
So I don't really think it was a mistake because it happened because he didn't learn the playbook and really wasn't a great fit for this system. I think they kept him because he does have some height and some potential, but he was #6 on the depth chart because they were trying to stash him to give him time to learn to run better routes and to learn the playbook. There is a reason 32 other teams let him clear waivers. He's an interesting developmental prospect but it is hard to keep a guy like that on your 53 man roster. Of course at this point we have plenty of space and a need for WRs so might as well bring back a guy with some potential that knows some of the playbook.