The book seemed to try to imply through Slayer that you needed "two souls" in order to do it, but... Didn't the last book just get through telling us that the idea that wolfkin had a "human" half and a "wolf" half was all just in Perrin's head, and was just him not being able to come to grips with himself???
So... Meh.
That's because it's not how it works.
Brandon refused to give the full answer for now (it appears more information on this will come from the Encyclopedia), but from the hints he's given wolfbrothers have a specific wolf to which they are soul bonded and through which bond they find themselves able to communicate with all the others wolves. In Perrin's case this spirit guide was Hopper and Hopper died in EOTW, but obviously his bond to Perrin's soul remains. That's the connection between Perrin and Luc/Isam: in both cases there's two souls involved and one is dead and the other is alive.
Forcibly making Slayer involved mimicking in some way the "dead in TAR" state of the HOH (if Luc isn't a Hero of the Horn, many believe Tigraine was Callian the Chooser and Luc was Shivan the Slayer) - Tigraine's choices lead to her to giving the world the Dragon - ushering the world to the end of the Age, and Slayer's destruction happened in the final moments before Rand dealt with the Bore, which could be the official mark of the beginning of the new Age).
Brandon also hinted in a Q&A that the bond to Perrin's soul probably spared Hopper "the final death" wolves would normally get from being killed in TAR.
It does not contradict anything about Perrin's realization that he being a wolfbrother doesn't mean you might lose your humanity and become a wolf. Perrin believed there was something of the wolf in himself, in his nature and soul, in fact he was soul bonded to a wolf. It can very well mean Perrin could do what we haven't seen him do, which would be to transform into a wolf as he jumps out of TAR, much the same way Slayer could adopt at will the body of living Isam or dead Luc.
