C<&PL_padname_undef>
and C<&PL_padname_const> are used is an implementation detail subject to
change. To test for them, use C<!PadnamePV(name)> and
-C<PadnamePV(name) && !PadnameLEN(name)>, respectively.
+S<C<PadnamePV(name) && !PadnameLEN(name)>>, respectively.
Only C<my>/C<our> variable slots get valid names.
The rest are op targets/GVs/constants which are statically allocated