$h->header(field[=> $val],...)
When called with just an HTTP header as a parameter, this method returns the
current value for the header. For example, $myobject->('content-type') would
return the value for the object's Content-Type header. To define a new header
value, invoke header with a hash of header=>value pairs,
where the value is a scalar or reference to an array. For example, to define
the Content-Type header, you would do this:
$h->header('content-type' => 'text/plain');