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      Naming conventions of session files are defined by
      $CGI::Session::Driver::file::FileName global variable.
      Default value of this variable is cgisess_%s, where %s
      will be replaced with respective session ID. Should you
      wish to set your own FileName template, do so before
      requesting for session object:

          $CGI::Session::Driver::file::FileName = "%s.dat";
          $s = new CGI::Session();

      For backwards compatibility with 3.x, you can also use the
      variable name $CGI::Session::File::FileName, which will
      override one above.