This event is signalled by act_dispatch() in inc/actions.php after preparing the headers and before loading the template.
$data is an array of header strings. There is one DokuWiki header 'Content-Type: text/html; charset=utf-8'. Handlers of this event can add/modify/delete the headers in the event data. The event data must remain an array, even if emptied.
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.