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.