In Unix, you can use crontab with something like:

0 20 * * fri /path/to/your/script.pl
[download]

In Win32, you can either have a script that sleeps "the correct amount of time" before activating, then it goes to sleep again, or you could roll your own service with Win32::Daemon (note: it's outside CPAN). I'd prefer a third way: swap the OS :)