Stop localised ties from becoming ro when COW

Author: Father Chrysostomos

mg.c

@@ -536,7 +536,7 @@ Perl_mg_localize(pTHX_ SV *sv, SV *nsv, bool setmagic)
 			    mg->mg_ptr, mg->mg_len);
 
 	/* container types should remain read-only across localization */
-	SvFLAGS(nsv) |= SvREADONLY(sv);
+	if (!SvIsCOW(sv)) SvFLAGS(nsv) |= SvREADONLY(sv);
     }
 
     if (SvTYPE(nsv) >= SVt_PVMG && SvMAGIC(nsv)) {

t/op/tie.t

@@ -1014,3 +1014,19 @@ print "ok\n";
 
 EXPECT
 ok
+########
+#
+# Localising a tied COW scalar should not make it read-only.
+
+sub TIESCALAR { bless [] }
+sub FETCH { __PACKAGE__ }
+sub STORE {}
+tie $x, "";
+"$x";
+{
+    local $x;
+    $x = 3;
+}
+print "ok\n";
+EXPECT
+ok