perldoc > getpgrp(2)

    getpgrp PID
            Returns the current process group for the specified PID. Use a
            PID of 0 to get the current process group for the current
            process. Will raise an exception if used on a machine that
            doesn't implement getpgrp(2). If PID is omitted, returns the
            process group of the current process. Note that the POSIX
            version of "getpgrp" does not accept a PID argument, so only
            "PID==0" is truly portable.

            Portability issues: "getpgrp" in perlport.