See the current Perl documentation for getpgrp.

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

getpgrp - get process group

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 process group of current process. Note that the POSIX version of getpgrp() does not accept a PID argument, so only PID==0 is truly portable.