#!/usr/bin/perl -w
use strict;
use POSIX qw{setsid};
### or just 'use POSIX;' if you plan to use other
### functions from it, of course

if ( fork ) {
    {
        ### grandparent code goes here
        print "My grandkid should do a cute l'il ls...\n";
    } 
    exit; ### die if we're the parent
}
if ( fork ) {
    exit; ### die if we're the parent of this one, too.
}
### grandchild code starts here
POSIX::setsid; ### should be part of a new session now
exec '/bin/ls', '.';