From: Jakub Wilk <jwilk@jwilk.net>
Date: Sat, 26 Jan 2019 17:48:21 +0100
Subject: [PATCH] Warn against suspicious two-argument open() calls.

$ perl -e 'print <>' /etc/issue
two-argument open(..., "/etc/issue") at -e line 1.
Debian GNU/Linux 9 \n \l

$ perl -e 'print <<>>' /etc/issue
Debian GNU/Linux 9 \n \l
---
 doio.c | 3 +++
 1 file changed, 3 insertions(+)

--- a/doio.c
+++ b/doio.c
@@ -491,6 +491,9 @@
     bool was_fdopen = FALSE;
     char *type  = NULL;
 
+    if (num_svs == 0 && oname[0] != '<' && oname[0] != '>')
+	Perl_ck_warner_d(aTHX_ packWARN(WARN_DEPRECATED), "two-argument open(..., \"%s\")", oname);
+
     PERL_ARGS_ASSERT_DO_OPEN6;
 
     /* For ease of blame back to 5.000, keep the existing indenting. */