$ua->cookie_jar([$cookie_jar_obj])
              Get/set the cookie jar object to use.  The only
              requirement is that the cookie jar object must implement
              the extract_cookies($request) and
              add_cookie_header($response) methods.  These methods
              will then be invoked by the user agent as requests are
              sent and responses are received.  Normally this will be
              a "HTTP::Cookies" object or some subclass.

              The default is to have no cookie_jar, i.e. never
              automatically add "Cookie" headers to the requests.
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