The method follow_link takes a hash:

$mech->follow_link( text => "download", n => 3 )
[download]

you are getting your error since you're passing one element: the url. In fact, you don't need to use follow_link, just use:

$mech->get($_->url);
[download]

If you're trying to download the images (looks like it from your regex), you may simply do:

$mech->mirror($_->url_abs, "some_local_file_name");
[download]