patch to restrict exim's `{$perl {...}}` expansion function for hardening purposes

exim-restrict-perl.patch

--- old/src/perl.c      2017-03-04 21:21:35.000000000 +0000
+++ new/src/perl.c      2019-03-09 23:42:10.546624662 +0000
@@ -160,6 +160,23 @@
   uschar *str;
   int items;
 
+  if (name[0] == '_')
+    {
+    *errstrp = US"forbidden function name";
+    return 0;
+    }
+
+  int i = 0;
+  while (name[i] > 0)
+    {
+    if (name[i++] == ':')
+      {
+      *errstrp = US"forbidden function name";
+      return 0;
+      }
+    }
+
+
   if (!interp_perl)
     {
     *errstrp = US"the Perl interpreter has not been started";