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/