Actually, PL_hints is just used at compile-time, not at runtime. But using (PL_curcop->op_private & HINT_STRICT_STRINGIFY)
instead should work.
UPDATE: Oh, and a minor note: Perl_warner needs an additional argument that indicates the warning category, like:
if ((PL_curcop->op_private & HINT_STRICT_STRINGIFY)
"Stringification of reference disallowed");
or alternatively you could omit the check and use Perl_warn which takes no such argument.