agv = gv_fetchpv("a", GV_ADD, SVt_PV);
GvMULTI_on(agv); /* No apparent effect. */

agv = gv_fetchpv("a", GV_ADD|GV_ADDMULTI, SVt_PV); /* Nope */
agv = gv_fetchpv("a", GV_ADDMULTI, SVt_PV); /* *sigh* No. */