sv_magic(SvRV(obj), obj, '~', NULL, 0);
sv_magic(SvRV(obj), Nullsv, '~', NULL, -1);
static void modperl_filter_mg_set(pTHX_ SV *obj, modperl_filter_t *filter){    sv_magic(SvRV(obj), obj, '~', NULL, 0);    SvMAGIC(SvRV(obj))->mg_ptr = (char *)filter;}