my $fork_period = 30*60; # 30 minutes
my $forktime = time + $forkperiod;
while (1) {
    # Do my happy processing.
    # This must loop periodically (at most once a minute)
    # otherwise you may badly miss your forktime and get 
    # killed.

    # We don't want to fork too often (or they might think
    # we are a runway and kill us).
    if (time > $forktime) {
        # Kill the parent
        exit if fork();

        $forktime = time + $forkperiod;
    }
}