warn on $a.$b.$c in void context

$a.$b.$c gets transformed early on to execute as ($a.$b).=$c, which didn't
warn about void context becuase .= looks like a useful side effect.
Happily, the recently-added OPpCONCAT_NESTED flag identifies that this
has happened.  Make scalarvoid() pay attention to this flag when a concat
op is put into void context.  Fixes [perl #6997]

1 files changed, 5 insertions, 0 deletions

diff --git a/op.c b/op.c
index 74de752405..402c6f1610 100644
--- a/op.c
+++ b/op.c
@@ -1949,6 +1949,11 @@ Perl_scalarvoid(pTHX_ OP *arg)
             if (o->op_type == OP_REPEAT)
                 scalar(cBINOPo->op_first);
             goto func_ops;
+	case OP_CONCAT:
+            if ((o->op_flags & OPf_STACKED) &&
+		    !(o->op_private & OPpCONCAT_NESTED))
+                break;
+	    goto func_ops;
         case OP_SUBSTR:
             if (o->op_private == 4)
                 break;