Kudos. That is it. Thank you.

my $ua = LWP::UserAgent->new;
$ua->agent("MyApp/0.1 ");

$res = $ua->post($url,
                 Content_Type => 'form-data',
                 Content      => [ "ac" => 'upload',
                                   "file" => [ "$FILE" ],
                                 ]
     ); 
print $res->content, "\n";
[download]