Fix t/051-fsfile.t cleanup on Windows
diff --git a/perl/t/051-fsfile.t b/perl/t/051-fsfile.t
index 6a7e34f..99c6dd5 100644
--- a/perl/t/051-fsfile.t
+++ b/perl/t/051-fsfile.t
@@ -97,6 +97,7 @@
 $dupe->read( $foo, 4 );
 
 is( $foo, 'afoo', "reopened instream" );
+undef $instream;
 
 # Trigger destruction.
 undef $folder;