Perl5 リファレンス

       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 POSIX setpgid(2) or BSD setpgrp(2).  If
               the arguments are omitted, it defaults to `0,0'.
               Note that the BSD 4.2 version of `setpgrp' does
               not accept any arguments, so only `setpgrp(0,0)'
               is portable.  See also `POSIX::setsid()'.