(Unrelated to your actual question.) PL_na is not much used these days. The perlapi documentation has this to say about it:
<quote>
PL_na A convenience variable which is typically used with "SvPV" when
one doesn't care about the length of the string.
It is usually
more efficient to either declare a local variable and use that
instead or to use the "SvPV_nolen" macro.