There are many problems with this code, but here are your biggies:

• Don't construct your query string by hand. The POST method of HTTP::Request::Common does this for you. You simply supply it with an anonymous array in the form of [username=>$username, password=>$password, ...].
• If you do the above, you will not need your substitution statement.
• The || operator is a high-precedence operator. See perlop for the details, but in short, the || in the "fail 1" line evaluates before the comma operator does. In fact, the only reason your code does not print "fail 1" is because || short-circuits on the "true" evaluation of $formurl.
• In general, what you're trying to do is actually much easier than you are making it out to be. Here is one way it can be done:

  use LWP::UserAgent;
  use HTTP::Request::Common;
  
  my $formurl  = "http://www.amihotornot.com/board.pl";
  my $id       = 20010131;
  my $username = "MeowChow";
  my $password = "catfood";
  my $subject  = "Fancy Feast is neither fancy nor a feast.";
  my $post     = "Discuss amongst yourselves...";
  
  # construct an anonymous array for the Http::Request::Common::POST met
  +hod
  my $formvars = [
    id        => $id,
    username  => $username,
    password  => $password.
    subject   => $subject,
    comments  => $post,
    submit    => "Submit",
  ];
    
  $ua = new LWP::UserAgent;
  $ua->request(POST $formurl, $formvars);
  [download]

• Read the HTTP::Request::Common documentation, specifically the part about POST.

   MeowChow                                               
                print $/='"',(`$^X\144oc $^X\146aq1`)[-2]