If you don't want to change any application code, you could have separate instances of Starman listening at different ports, and then have a user-facing Apache instance that runs a reverse proxy for each subdomain.

Or you could have a user-facing ngix/lighthttpd/perlbal instance that runs a reverse proxy for each subdomain :) Plack::Middleware::ReverseProxy

example reverse proxy config for apache/apache2/ngix/lighthttpd http://www.omakase.org/perl/starmanreverseproxyab.html

Perlbal / http://wiki.catalystframework.org/wiki/deployment/perlbal-starman-psgi