HTTP::Server::PSGI versions before 1.0056 for Perl allow a single request to crash the server by declaring a large Content-Length.
The request body is read with sysread() calls sized by the remaining Content-Length. Perl grows the read buffer to the requested length before any data arrives, so the server tries to allocate as much memory as the client declares.
A remote attacker can send one request with a large Content-Length and a few body bytes, and perl exits with an out-of-memory error before the application is called. HTTP::Server::PSGI runs as a single process, so the server stays down until restarted.
Upgrade to Plack 1.0057 or later.