use LWP::Simple qw(!head);
use LWP::UserAgent;
  use HTML::LinkExtor;
  use URI::URL;

  my $ua = LWP::UserAgent->new;
  my $p = HTML::LinkExtor->new;
  $ua->timeout(3);
  my $res = $ua->request(HTTP::Request->new(GET => $url),
                      sub {$p->parse($_[0])});

foreach(@url)
{

  $url = "$_";

  my $req = HTTP::Request->new(POST => "$url");
  $req->content_type('application/x-www-form-urlencoded');
  $req->content('query=libwww-perl&mode=dist');
  my $res = $ua->request($req);

  if ($res->status_line =~ /404/ || $res->status_line =~ /401/)
      { 
       print "FAILED!";
      }
  elsif ($res->status_line =~ /408/)
      {
         print "TIMED OUT";
      }
  else 
  {
  print "PASSED!!"
  }
}
[download]

I see what the problem is now after posting your code. LWP returns a status of 500 if a timeout occurs, not the 408 you are looking for.

To catch your errors, you could try

if ($res->success) {
    print "PASSED!\n";
} else {
    print "An error occured:  " . $res->status_line . "\n";
}
[download]

Thanks for your help!! This is very promising!