keep in mind that PCRE isn't really "Perl Compatible"

I don't find this distinction to be helpful. PCRE is Good Enough. Of course truly compatible Perl regular expressions would allow you to execute arbitrary Perl code, and PCRE doesn't. I cannot imagine why people should be surprised by this.

When I'm building patterns for Postfix, I'm not particularly fussed by the absence (quite the opposite, in fact). I'm pleased the rest of the functionality is available and that I can avoid POSIX syntax. A pattern that can be elegantly expressed using a positive zero-width lookbehind assertion and/or a negative zero-width lookahead assertion is usually impossible to write for an RE engine that lacks them. That you can't execute Perl code is a comparatively minor inconvenience.

Having Perl patterns execute Perl code won't be truly useful until the code can do its own pattern matching in turn. Last time I looked, the RE engine wasn't reentrant, or whatever, precluding one from doing such things. It's still an experimental proposition.