See the current Perl documentation for setpgrp.

Here is our local, out-dated (pre-5.6) version:

setpgrp - set the process group of a process

setpgrp PID,PGRP

Sets the current process group for the specified PID, 0 for the current process. Will produce a fatal error if used on a machine that doesn't implement setpgrp(2). If the arguments are omitted, it defaults to 0,0. Note that the POSIX version of setpgrp() does not accept any arguments, so only setpgrp 0,0 is portable.