Note that some bits may be relevent at compile-time only, some at
run-time only. This is a new mechanism and the details may change.
+=item $^R
+
+The result of evaluation of the last successful L<perlre/C<(?{ code })>>
+regular expression assertion. (Excluding those used as switches.) May
+be written to.
+
=item $^S
Current state of the interpreter. Undefined if parsing of the current