To address one question:

...limitations on what can be an argument to Regexp Quote Like Operators

# while ( my $rv = $$str_ref =~ m{$pat_re}gc ) { does not work

But it does. All tests in the suite pass OK with this line restored. You probably used the same scalar variable with other (not shown) tests in succession, and the (useless here) "c" modifier had left this scalar's pos property being set after the very 1st splitF call, then further calls produced bogus results. So, the bad (or good?) news is, there is no undocumented limitation, but the really good news -- in this case, I think -- if you had omitted "c" but wrote line as above, all tests (shown or not) would pass OK. And yet, such implementation would lead to subtle and evil bugs: the pos of referent (splitF argument, as opposed to disposable lexical copy) would be: (a) reset, and, maybe worse, (b) taken into account if it had been set previously to splitF call.