Perl LWP 403 ?

So yeah.. here's the script

#!/usr/bin/perl

use LWP::UserAgent; $ua = new LWP::UserAgent;
my $req = new HTTP::Request('POST','http://img.4chan.org/b/res/1234567890.html';); # random
$req->content_type('application/x-www-form-urlencoded');
$req->content('com=iamtesting');
my $res = $ua->request($req);
print $res->as_string;

I get a 403.
The thread exists though.
what am i doing wrong ?
I checked and "com" is the comment variable.

So ok my script now is ..
-----
#!/usr/bin/perl

use LWP::UserAgent;
$ua = new LWP::UserAgent;
$ua->agent("Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.0.9) Gecko/20070126 Ubuntu/dapper-security Firefox/1.5.0.");
my $req = new HTTP::Request('POST','http://dat.4chan.org/b/imgboard.php';); # random
$req->content_type('application/x-www-form-urlencoded');
$req->content('resto=20112038com=YHALOTHAR');
my $res = $ua->request($req);
print $res->as_string;
----

It doesn't work.. I figured out that resto= is the thread number, and com is for the comment..
But what is wrong? It still doesn't work