If you're using 1.4.22 or older version of lighttpd and mount your
application in the root path, you should enable LighttpdScriptNameFix
middleware, or upgrade to 1.4.23 and include 'fix-root-scritpname'
flag.
via github.com
A new developer release of Plack is on CPAN, with an incompatible fix for FastCGI on lighttpd.
Your application will be affected if you mount your application under the root ("/") with lighttpd 1.4.22 or older, or with 1.4.23 without fix-root-scriptname option. See the issue and perldoc Plack::Handler::FCGI for details how to configure it correctly.