You aren't trying to do a POST you are trying to do a GET, which maybe the limiting factor, though I suspect it is more likely that there is some characters in @message that is causing the proplem. Try either:

use URI::Escape;
my $message = uri_escape(join('', @message));
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Or, more sensibly, a POST request:

use HTTP::Request::Common qw(POST);
use LWP::UserAgent;
my $url = 'http://xxx.xxxx.xxx/somewhat/bla/bboard/newthread.php';
my $ua = LWP::UserAgent->new;
my $res = $ua->request(POST $url, [action => 'send', boardid => 4]);
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Hope this helps.

gav^