According to perlvar for $):

The first number sets the effective gid, and the rest (if any) are passed to setgroups(). To get the effect of an empty list for setgroups(), just repeat the new effective gid

So:

$) = "$gid $gid";
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The following code on my machine (NetBSD):

use strict;
use warnings;
use POSIX;
my( $uid, $gid ) = qw/32767 32766/;
setgid( $gid ); $) = "$gid $gid"; $( = $gid; die "Set group ($gid): $!
+"
  if ( $( != $gid ) or ( $) != $gid );
setuid( $uid ); $> = $uid; $< = $uid; die "Set user ($uid): $!"
  if ( $< != $uid ) or ( $> != $uid );
print `id`;
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Produces:

uid=32767(nobody) gid=32766(nogroup) groups=32766(nogroup)
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