It's not unlike the call to open failed, but your script failed to notice and act accordingly e.g. by dieing.

Try, at line 2, inserting

use warnings;
use strict;
use autodie;
[download]

. These 3 lines will have the effect of

• giving you a heads-up on any potential problems,
• telling you of any scripting errors and finally,
• causing the script to die with an explanation if a system call e.g. the aforementioned open(), fails for any reason.