Plack is the Perl implementation of the PSGI (Perl Web Server Gateway Interface) specification, analogous to Python's WSGI or Ruby's Rack. It provides a standardized interface between web application frameworks and web servers, along with a rich ecosystem of middleware components, server adapters, and utility modules. The codebase is a well-structured library rather than an end-user application, with the primary entry point being the `plackup` script for running PSGI applications from the command line. Key architectural components include `Plack::Handler` subclasses for various server backends (CGI, FastCGI, Apache, standalone), `Plack::Middleware` base class for composable request/response filters, and `Plack::App` for serving static files, CGI scripts, and URL routing. The system supports both synchronous and streaming responses, and includes comprehensive testing utilities via `Plack::Test`. The code is mature, actively maintained, and demonstrates strong Perl engineering practices with clean object-oriented design, extensive test coverage, and reusable patterns.