if (!fork) { # doing stuff in child process...
  # open your file handle here
  while (my $bytesread = read($filesource, my $buffer,1024)) 
  {
    print OUTFILE $buffer;
  } 
  # you could also write to a 'status' file and display that back to the client ;)
  exit; 
}

print $cgi->header();
print "Your upload is in process... see this status.html page for the status (changes every once in awhile :)\n";
exit;