Perps were actually Post Retirement Pilots, or PRP's, pronounced Perp.

Most (but not all) of them were line check airmen and wide body Capts. The logic to allowing them to "Retire" but continue to fly, was because so many LCA's retired, they were short. So the LCA's were allowed to perform IOE's and TOE's, until their replacements were trained.

I think it took about a year for the longest serving ones to finally be "Retired", but in the mean time, the Perps were flying a whole bunch of green slips!