Just for the record, Text::CSV_XS does know the difference between NULL/undef and empty strings. This will show when using always_quote => 1:

   my $csv = Text::CSV_XS->new ({ always_quote => 1});
   $csv->combine ("", 1, undef, "bar");
   print $csv->string, "\r\n";

Will result in:

   "","1",,"bar"

Note the missing quotes for the undef entry

If requested, I can consider adding an option to do the reverse also: when always_quoted is true, return empty columns as undef in the $csv->fields call.