but I'm curious what any other monks have to say or suggest about this feature. Can it still be something that I keep in my toolkit, or should I just assume that I'll need to stick with (?PARNO) from now on when I want recursion in a regex.

the (??{ code }) feature is available since perl 5.8, and still marked as experimental in 5.12. That's extraordinarily long, and the reason is that the feature did exhibit a multitude of problems. For example in 5.10 it wouldn't recognize say even if the feature was enabled, would segfault when you execute too much code in it and then reenter the regex engine, and had some nasty scoping bugs.

What I want to say is that it's really still experimental and unreliable. Whenever you've got the chance to use the (?PARNO) syntax instead, you should do it.