config.yml

[davidp@supernova:~]$ cat test.yml 
plack_middlewares:
     Static:
       - path
       - qr{^/(images|js|css)/}
       - root
       - './public/'

serialises to:

{
  plack_middlewares => {
        Static => ["path", "qr{^/(images|js|css)/}", "root", "./public/"],
      },
}