make $1 et al readonly under threads; make C<undef $1> fail like

C<$1 = undef> does

p4raw-id: //depot/perl@2335

1 files changed, 5 insertions, 2 deletions

diff --git a/pp.c b/pp.c
index 2686116698..07b83dba3d 100644
--- a/pp.c
+++ b/pp.c
@@ -762,8 +762,11 @@ PP(pp_undef)
 	RETPUSHUNDEF;
 
     if (SvTHINKFIRST(sv)) {
-	if (SvREADONLY(sv))
-	    RETPUSHUNDEF;
+	if (SvREADONLY(sv)) {
+	    dTHR;
+	    if (PL_curcop != &PL_compiling)
+		croak(PL_no_modify);
+	}
 	if (SvROK(sv))
 	    sv_unref(sv);
     }