Your script works as advertised, and the fact that it's wordier than necessary is obviously deliberate, so that beginners can follow it more easily. The one problem I *do* have with it is that there is no-reason to hard-code to number of elements in each list. Much better to use the $# construct to return the size of the list. In other words:

print "@groups[rand($#groups + 1)]\n";
[download]

will work even if you add or remove members from @groups.