$ua->default_header( $field => $value )

    This is just a short-cut for $ua->default_headers->header( $field => $value ). Example:

      $ua->default_header('Accept-Encoding' => scalar HTTP::Message::decodable());
      $ua->default_header('Accept-Language' => "no, en");

##</code><code>##

$client->transport->default_header( 'my_special_header' => 'sha1hash');

##</code><code>##

request_preprepare => sub { my($request, $ua, $h) = @_; ... }

    The handler is called before the request_prepare and other standard initialization of the request. This can be used to set up headers and attributes that the request_prepare handler depends on. Proxy initialization should take place here; but in general don't register handlers for this phase.

##</code><code>##

$client->transport->default_header( 'properly-speleeed-key' => $value );