Handle cow $_ in @INC filter

Setting $_ to a copy-on-write scalar in an @INC filter causes the
parser to modify every other scalar sharing the same string buffer.
It needs to be forced to a regular scalar before the parser sees it.

1 files changed, 6 insertions, 1 deletions

diff --git a/t/op/incfilter.t b/t/op/incfilter.t
index 582b691d82..6227c4ac37 100644
--- a/t/op/incfilter.t
+++ b/t/op/incfilter.t
@@ -13,7 +13,7 @@ use strict;
 use Config;
 use Filter::Util::Call;
 
-plan(tests => 144);
+plan(tests => 145);
 
 unshift @INC, sub {
     no warnings 'uninitialized';
@@ -216,6 +216,11 @@ do [\'pa', \&generator_with_state,
      "pass('And return multiple lines');\n",
     ]] or die;
 
+@origlines = keys %{{ "1\n+\n2\n" => 1 }};
+@lines = @origlines;
+do \&generator or die;
+is $origlines[0], "1\n+\n2\n", 'ink filters do not mangle cow buffers';
+
 # d8723a6a74b2c12e wasn't perfect, as the char * returned by SvPV*() can be
 # a temporary, freed at the next FREETMPS. And there is a FREETMPS in
 # pp_require