I can't point you to any documents or sample code right now but, in Cat at least, they are arbitrary strings which are parsed out. "Unrecognized" ones would only be fatal in this context if you intentionally made misparsing of them fatal. I have mixed feelings about them because it is just string wrangling here. They are very useful but seem clumsy (you can stack up a bunch of them for one controller sub) and I think it might be nice if the new Moose stuff would mutate them into some other syntax eventually.