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.