Plack::Middleware::Precompressed is an alternative (or rather, complement) to middlewares like Deflater, which will compress response bodies on the fly. For dynamic resources, that behaviour is necessary, but for static resources it is a waste: identical entities will be compressed over and over. Instead, Precompressed allows you to compress static resources once, e.g. as part of your build process, and then serve the compressed resource in place of the uncompressed one for compression-enabled clients. To do so, it appends a .gz suffix to the request URI and tries to serve that. If that fails, it will try again with the unmodified URI.