I guess the next question is: are there any examples you want to match that do not start m{ ^ \s* sub \b }x? If not, then you have your answer; if there are, those are the tricky cases we'd need to see.

I just took a quick look at one of my codebases and found 558 matches for that pattern; 503 of them were normal sub declarations all matching m{ ^ \s* sub \s+ (\w+) \s* \{ }x, the rest were anonymous sub references not matching that pattern. Of matches against m{ \b sub \b }x elsewhere in any line, almost all were anonymous sub references (the remaining 2 or 3 were comments or $sub variables).

So that pattern would work for me, would it work for you?