It also suffices in recent LWP releases to simply say:

my $ua = LWP::UserAgent->new(max_size => 1);
...
for my $url (@list_of_urls_to_check) {
  my $res = $ua->head($url);
  unless ($res->is_success) {
    $res = $ua->get($url);
  }
  ...
}
[download]

which effectively does what your callback does in a lot less typing. {grin}