$response is an object of class HTTP::Response. If you read that documentation, you will find the following info:

$r->header( $field )
$r->header( $field => $value )

This is used to get/set header values and it is inherited from HTTP::Headers via HTTP::Message. See HTTP::Headers for details and other similar methods that can be used to access the headers.

If you check out HTTP::Headers, you will find more methods that are implied by the above to work too, for example $h->header_field_names, which returns a list of all header names in the response.