Wikipedia
PSGI or Perl Web Server Gateway Interface is an interface between web servers and Perl-based web applications and frameworks that allows writing portable applications that can be run as standalone servers or using CGI, FastCGI, mod_perl, et al. It is inspired by the Web Server Gateway Interface for Python, Rack for Ruby and JSGI for JavaScript.
A PSGI application is a Perl subroutine that accepts arguments as a single hash reference and returns a reference to an array of three elements: an HTTP status code, a reference to an array of HTTP headers and a reference to an array of HTTP body lines (usually a generated HTML document) or a filehandle-like object.
Plack is a reference PSGI implementation.
Web frameworks with PSGI support: