PERP stands for 'Pulsar Effect Role Play'. Pulsareffect was an ancient Garry's Mod community which, as the name suggests, were the initial developers of the PERP gamemode. . Pulsar Effect began licensing the gamemode to other communities. Hellzoneclan being one of them. Killslick was the owner of hellzone and bought a license to use PERP, like many other servers.
PERP used to be a very popular gamemode, but many servers died because a dozen servers with an identical gamemode aren't going to last. Servers that innovated with custom development survived for much longer which is the reason why when you search for the PERP source code, you'll find many versions of it, like PERP2, PERP2,5, PERP3, PERPX, gtaonline, etc, etc...
During the time that PERP servers were starting up left and right with and without licenses,
Killslick registered an US patent with the patent office for this 'code'. Just because you have a registered patent, doesn't mean anything, but killslick being a weird adult with adult money had enough legal power to threaten usually young, unknowing (aspiring) server owners. You can't DMCA, or patent, a software work made and authored by different people, especially derivative works.
For all these BOGUS DMCA claims killslick made a name of himself and considering his age he became quite a target of doxing and pedo allegations. A huge lot of this history is lost because most gaming forums have now shut down, but if you dig very deep you can still find alot about him. Dude's probably in his 60's already. I have a copy of his patent somehwere, having been DMCA'd myself lol.