Plack::Middleware::Expires is Apache's mod_expires for Plack. This middleware
controls the setting of Expires HTTP header and the max-age directive of the
Cache-Control HTTP header in server responses. Note: Expires works only for
successful response and If exists Expires HTTP header already, this middleware
does not override.

WWW: http://search.cpan.org/dist/Plack-Middleware-Expires/