The other annoying warning is "Name ... used only once: possible typo...". For this I do this song-and-dance:

  $DB::single = $DB::single = 1;

##</code><code>##

    use DB_File;
    # Just some global in DB_File.
    $DB_File::db_version = 42;

##</code><code>##

    use DB_File;
    $DB_File::wibble = 42;

##</code><code>##

    require DB_File;
    $DB_File::db_version = 42;