patch 8.2.0672: heredoc in scripts does not accept lower case marker

Problem:    Heredoc in scripts does not accept lower case marker.
Solution:   Allow lower case only in non-Vim scripts. (Ken Takata,
            closes #6019)
diff --git a/src/testdir/test_perl.vim b/src/testdir/test_perl.vim
index 35cb813..1a856d5 100644
--- a/src/testdir/test_perl.vim
+++ b/src/testdir/test_perl.vim
@@ -305,7 +305,10 @@
   perl << trim
     VIM::DoCommand('let s ..= "D"')
   .
-  call assert_equal('ABCD', s)
+  perl << trim eof
+    VIM::DoCommand('let s ..= "E"')
+  eof
+  call assert_equal('ABCDE', s)
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab