See the current Perl documentation for getpgrp.
Here is our local, out-dated (pre-5.6) version:
getpgrp - get process group
Returns the current process group for the specified
PID. Use a
0 to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement
PID is omitted, returns process group of current process. Note that the
POSIX version of
does not accept a
PID argument, so only
PID==0 is truly portable.