Does the forum depend on Cookies? If so, you'll need to set up a cookie jar so that your redirector can capture them and issue them back with your post.

Something like the following is the general idea:

use HTTP::Request::Common qw(GET POST);
use HTTP::Cookies;
use LWP::UserAgent;
...
my $ua = new LWP::UserAgent();
my $cookies = new HTTP::Cookies(file => 'cookies.dat',
                                autosave => 1);
$ua->cookie_jar($cookies);

# We now have a persistent cookie jar associated with the
# user agent.

my $req = GET "$url";

# if the response included a cookie, it'll be saved in the
# jar, and resent along with subsequent GETs or POSTs to
# the same domain that issued the cookie.
[download]

See HTTP::Cookie for more details.

Assuming the forum requires that you log in, you'll need to manually simulate the login steps so that you can capture a login cookie. WWW::Mechanize can help there.