-# define PmopSTASHPV(o) ((o)->op_pmstashpv)
-# define PmopSTASHPV_set(o,pv) (PmopSTASHPV(o) = savesharedpv(pv))
+# define PmopSTASHPV(o) \
+ (((o)->op_pmflags & PMf_ONCE) ? (o)->op_pmstashstartu.op_pmstashpv : NULL)
+# if defined (DEBUGGING) && defined(__GNUC__) && !defined(PERL_GCC_BRACE_GROUPS_FORBIDDEN)
+# define PmopSTASHPV_set(o,pv) ({ \
+ assert((o)->op_pmflags & PMf_ONCE); \
+ ((o)->op_pmstashstartu.op_pmstashpv = savesharedpv(pv)); \
+ })
+# else
+# define PmopSTASHPV_set(o,pv) \
+ ((o)->op_pmstashstartu.op_pmstashpv = savesharedpv(pv))
+# endif