You're incorrectly using the method. The referenced array should provide "the key/value pairs for the fill-in form content." I think you want the following variant:

$ua->post( $url, $field_name => $value,... Content => $content )
[download]

So

$req = $ua->post(
    $myurl,
    @netscape_like_headers,
    'Content-Type' => 'application/json; charset=utf-8',
    'Content'      => $postcontent,
);
[download]