• You're not rewinding your directory handle
• You're grabbing basenames in @files, not paths, so GoProcess might be confused.
• Might be better written as an infinite loop:

  while (1) {
    rewinddir(DIR);
    foreach my $file (grep -f, map "$dir/$_", readdir DIR) {
      GoProcess($file);
    }
    sleep 30;
  }
  [download]

• Your closedir is never reached. When do you want it to stop?

-- Randal L. Schwartz, Perl hacker