Web::Simple versions before 0.035 for Perl allocate a buffer sized by the Content-Length header for urlencoded body parameters.
Web::Dispatch::ParamParser::get_unpacked_body_from reads an application/x-www-form-urlencoded body using a single read call, with no cap and the return value ignored. Perl's read grows the buffer to the requested length before reading, so the declared Content-Length determines the allocation.
An attacker can trigger this through any route whose dispatch specification has a % body-parameter match, such as sub (POST + %name=), sending a Content-Length far larger than the body actually sent. No authentication is involved.
For deployments that cannot upgrade but are running behind a reverse proxy, configure the proxy to reject requests above a maxiumum Content-Length.
Upgrade to Web::Simple 0.035 or later, and in the application configure $Web::Dispatch::ParamParser::MAX_BODY_SIZE.