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

...

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

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

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

    The handler is called before the request_prepare and other standar
+d initialization of the request. This can be used to set up headers a
+nd attributes that the request_prepare handler depends on. Proxy init
+ialization should take place here; but in general don't register hand
+lers for this phase.

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