reverted:
--- b/perl.h
+++ a/perl.h
@@ -5558,7 +5558,14 @@
 EXTCONST runops_proc_t PL_runops_dbg
   INIT(Perl_runops_debug);
 
+/* PERL_GLOBAL_STRUCT_PRIVATE wants to keep global data like the
+ * magic vtables const, but this is incompatible with SWIG which
+ * does want to modify the vtables. */
+#ifdef PERL_GLOBAL_STRUCT_PRIVATE
+#  define EXT_MGVTBL EXTCONST MGVTBL
+#else
+#  define EXT_MGVTBL EXT MGVTBL
+#endif
-#define EXT_MGVTBL EXTCONST MGVTBL
 
 #define PERL_MAGIC_READONLY_ACCEPTABLE 0x40
 #define PERL_MAGIC_VALUE_MAGIC 0x80