perldoc > setpriority(2)

    setpriority WHICH,WHO,PRIORITY
            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.