According to Nginx goes beyond its server roots and launches its application platform
..Unit is a modern application server for applications written in PHP, Python and Go (with support for node.js, Java, Ruby Perl and others following soon), that run in dynamic environments.
Where does Unit fit in a ngnix->PSGI->Starman->Dancer stack? which part does it replace or adds to?
In your example nginx and Starman are comunicating using HTTP, not PSGI (because PSGI is interface, and HTTP is protocol). So stack will look like Unit->Dancer. In case if you need some nginx features your can use it in front of Unit: nginx->Unit->Dancer.