Presumably the original server process uses exec() to start the new process, which inherits the open socket handles, and the descriptor is passed as an argument

Presumably the original server process uses exec() to start the new process, which inherits the open socket handles, and the descriptor is passed as an argument to the new process. (e.g. http://stackoverflow.com/questions/14351147/perl-passing-an-open-socket-across-fork-exec http://stackoverflow.com/questions/14351147/perl-passing-an-...)