I think it clearest if I give an example script.

#!/usr/bin/perl -w
use strict;
use POSIX qw(setgid setsid setuid);

my $i;
my $pid;
my @user;
my $dmon_id="furrypop";
my $outfile="/export/home/furrypop/daemon.out";

&write_log("Start");

chdir '/' or die "Can't chdir to /: $!";
open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!";
defined($pid = fork) or die "Can't fork: $!";
if ($pid) {
    # parent-only processing follows...
    &write_log("Parent");
    sleep 10;
}
else {
    # child-only processing follows...
    # switch the ID under which the child is running
    setsid  or die "Can't start a new session: $!";
    @user=getpwnam($dmon_id);
    if (@user) {
#        setuid $user[2] or die "Cannot switch ID to $dmon_id: $!";
#        setgid $user[3] or die "Cannot switch group for $dmon_id: $!"
+;
        $< = $user[2];
        $> = $user[2];
        $( = $user[3];
        $) = $user[3];
    }
    else {
        die "Invalid user ID ($dmon_id)";
    }

    &write_log("Child");
    sleep 10;
}


sub write_log {
    open(OUTFILE, ">>$outfile") or die "Cannot open file $outfile : $!
+";
    print OUTFILE "$_[0] pid $$\n  user real $< effective $>\n  group 
+real $( effective $)\n";
    close OUTFILE;
}
[download]

Both the setuid/setgid and the assignation-to-special-variables code is shown above, the former commented out.

There's also a sleep in there in order to give one time to check the processes with ps.

With:-

$< = $user[2];
$> = $user[2];
$( = $user[3];
$) = $user[3];
[download]

The processes are:-

    root 15569  8068  0 16:06:31 pts/12   0:00 /usr/bin/perl -w daemon
+_gid_test.pl
furrypop 15570 15569  0 16:06:31 ?        0:00 /usr/bin/perl -w daemon
+_gid_test.pl
[download]

The output file contains:-

Start pid 15569
  user real 0 effective 0
  group real 1 12 9 8 7 6 5 4 3 2 0 1 effective 1 12 9 8 7 6 5 4 3 2 0
+ 1
Parent pid 15569
  user real 0 effective 0
  group real 1 12 9 8 7 6 5 4 3 2 0 1 effective 1 12 9 8 7 6 5 4 3 2 0
+ 1
Child pid 15570
  user real 514 effective 514
  group real 1 12 9 8 7 6 5 4 3 2 0 1 effective 1 12 9 8 7 6 5 4 3 2 0
+ 1
[download]

With:-

setuid $user[2] or die "Cannot switch ID to $dmon_id: $!";
setgid $user[3] or die "Cannot switch group for $dmon_id: $!";
[download]

The processes are:-

    root 15588  8068  0 16:08:50 pts/12   0:00 /usr/bin/perl -w daemon
+_gid_test.pl
    root 15589 15588  0 16:08:50 ?        0:00 /usr/bin/perl -w daemon
+_gid_test.pl
[download]

The output file contains:-

Start pid 15588
  user real 0 effective 0
  group real 1 12 9 8 7 6 5 4 3 2 0 1 effective 1 12 9 8 7 6 5 4 3 2 0
+ 1
Parent pid 15588
  user real 0 effective 0
  group real 1 12 9 8 7 6 5 4 3 2 0 1 effective 1 12 9 8 7 6 5 4 3 2 0
+ 1
Child pid 15589
  user real 514 effective 0
  group real 350 12 9 8 7 6 5 4 3 2 0 1 effective 1 12 9 8 7 6 5 4 3 2
+ 0 1
[download]