I highly doubt that the get routine is exported from WWW::Mechanize. Further, you're assigning back to your $webcrawler, so you're going to lose your object created by WWW::Mechanize->new(). So try something like:

  my $page = $webcrawler->get($url_name);
[download]

At least, that's the guess I have for not having used, or even installed, W::M.