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?