# Load different modules depending on OS
    if($operating_system =~ /^MSWin/i)
      {
        if(!&require_if_present("Win32"))
          {
            $no_win32 = 1;
          }
      }

    # The $no_tk etc vars are set by command line
    # options before I get here
    if(!$no_tk)
      {
        if(&require_if_present("Tk"))
          {
            &require_if_present("Tk::DialogBox");
            if($no_fileselect || 
                         !&require_if_present("Tk::FileSelect"))
              {
                $no_fileselect = 1;
              }
            if($no_progress || 
                         !&require_if_present("Tk::ProgressBar"))
              {
                $no_progress = 1;
              }
            if($no_dirtree || !&require_if_present("Tk::DirTree"))
              {
                $no_dirtree = 1;
              }
          }
        else
          {
            $no_tk = 1;
          }
      }

    # Then later I can...
    if($no_tk)
      {
        # Run without an interface
        ...
      }
    else
      {
        $main_win = new Tk::MainWindow(
            -title   => "My App",
          );
        ...
      }