You need to pass fields as an anonymous array after the url, like:

  use HTTP::Request::Common qw(POST);
  use LWP::UserAgent;
  $ua = LWP::UserAgent->new;

  my $req = POST 'https://rt.cpan.org/Public/Dist/Display.html',
                [ Status => 'Active', Name => 'libwww-perl' ];

  print $ua->request($req)->as_string;
[download]

See POST at lwpcook for the example.

I think you can also consider WWW::Mechanize to issue your POST

L*