setpriority
See the current Perl documentation for setpriority.
Here is our local, out-dated (pre-5.6) version:
setpriority - set a process's nice value
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user. (See
setpriority(2).)
Will produce a fatal error if used on a machine that doesn't implement
setpriority(2).