=over

=item $REAL_GROUP_ID

=item $GID

=item $(

The real gid of this process.  If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
list of groups you are in.  The first number is the one returned by
getgid(), and the subsequent ones by getgroups(), one of which may be
the same as the first number.

However, a value assigned to C<$(> must be a single number used to
set the real gid.  So the value given by C<$(> should I<not> be assigned
back to C<$(> without being forced numeric, such as by adding zero.

You can change both the real gid and the effective gid at the same
time by using POSIX::setgid().  Changes to $( require a check to $!
to detect any possible errors after an attempted change.

(Mnemonic: parentheses are used to I<group> things.  The real gid is the
group you I<left>, if you're running setgid.)

=back