If you want to get the error change your code to this:

open (UPLOAD, "+>$user_name" ) || print_error_page ("Can't open your s
+ubmit directory.<br>$!"); 
flock(UPLOAD, $LOCK_EX) || print_error_page("Can't flock your submit d
+irectory.<br>$!");
[download]

The $! will print out the error for you.