Bodge to make Tk work - like the UTF8 flag, the READONLY flag has too

many meanings. const const vs mutable in this case.

p4raw-id: //depot/perl@23085

sv.c

@@ -3941,7 +3941,13 @@ Perl_sv_utf8_upgrade_flags(pTHX_ register SV *sv, I32 flags)
 	return 0;
     if (!SvPOK(sv)) {
 	STRLEN len = 0;
-	(void) SvPV_force(sv,len);
+	if (SvREADONLY(sv) && (SvPOKp(sv) || SvIOKp(sv) || SvNOKp(sv))) {
+	    (void) sv_2pv_flags(sv,&len, flags);
+	    if (SvUTF8(sv))
+		return len;
+	} else {
+	    (void) SvPV_force(sv,len);
+	}
     }
 
     if (SvUTF8(sv)) {