Yes, the /z modifier would precisely fit what I'm looking for.

Let me clarify my problem by contrasting it with your sample code:

• The stream should consist exclusively of a sequence of matchable texts. That is, $` should always be ''. (Remember the ^ at the beginning of my $pattern.)
• I want to know as soon as possible that a pattern won't be able to match the buffer. It's somewhat easier for me because I know that my pattern begins with ^.

Dare I open up the perl source code and try to implement this now?

I poked around in regexec.c. Whoa, heavy magic! Maybe --just maybe-- if the perl RE engine weren't so optimized, I would have the confidence to change stuff and implement /z. But not on the existing RE engine.

I'm now considering using Sandfly's suggested technique with YAPE::Regexp to provide the list of atoms.

Thanks to all for the useful comments and suggetions.