Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Raises an exception when used on a machine that doesn't implement setpriority(2).
WHICH can be any of PRIO_PROCESS, PRIO_PGRP or PRIO_USER imported from "RESOURCE CONSTANTS" in POSIX.
Portability issues: "setpriority" in perlport.