I always check for user input and the (non)existence of it, so I added simple:

my $url = shift;
if (!$url) {
    print "Usage: $0 URI\n";
    exit 1;
}
[download]

Also if user enters URI without 'http', add it:

if ($url !~ m(^http://)i) {
    $url = 'http://' . $url;
}
[download]

Checking and validating user input is (for me) a must-have task, even for simple and small program like this.