patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)

src/testdir/test_perl.vim

@@ -219,11 +219,11 @@ endfunc
 
 func Test_stdio()
   redir =>l:out
-  perl <<EOF
+  perl << trim EOF
     VIM::Msg("&VIM::Msg");
     print "STDOUT";
     print STDERR "STDERR";
-EOF
+  EOF
   redir END
   call assert_equal(['&VIM::Msg', 'STDOUT', 'STDERR'], split(l:out, "\n"))
 endfunc
@@ -290,3 +290,22 @@ func Test_set_cursor()
   normal j
   call assert_equal([2, 6], [line('.'), col('.')])
 endfunc
+
+" Test for various heredoc syntax
+func Test_perl_heredoc()
+  perl << END
+VIM::DoCommand('let s = "A"')
+END
+  perl <<
+VIM::DoCommand('let s ..= "B"')
+.
+  perl << trim END
+    VIM::DoCommand('let s ..= "C"')
+  END
+  perl << trim
+    VIM::DoCommand('let s ..= "D"')
+  .
+  call assert_equal('ABCD', s)
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab