Is this a CGI script? Check your server's error log. You'll probably find an answer there.

Also, change your $ftp->get(.....) line as follows:

$ftp->get("00000001.txt", $filehandle)
    or die "Couldn't get 00000001.txt\n$!";
[download]

Then run the script again, and check your error logs. Also, if this is a CGI script, use CGI::Carp with the fatalsToBrowser setting. See the documentation for CGI::Carp at the link provided in this paragraph. Use it as a development tool, and then remove it once things are working as they should.

Most likely problems could include a problem getting the file, or a problem opening for output.