use POSIX ":sys_wait_h";

my %kids = ();

# Signal Handler for SIGCHLD
sub sigchld_handler{
    while (($pid = waitpid(-1, &WNOHANG)) > 0) {
        delete $kids{$pid};
    }
    $SIG{CHLD} = \&sigchld_handler;
}
$SIG{CHLD} = \&sigchld_handler;

# You can repeat this for as many secondary
# processes as you need to merge the old indexes
for (1..3) {
        if (my $pid = fork) {
            # Parent Process keep track of forked children
            $kids{$pid} = 1;
        }
        else {
            # Child process
            # ... do merging of old indexes here
            sleep(3);
            exit 0;   # MUST EXIT HERE
        }
}

while (keys %kids > 0) {
     sleep(1);  # wait for all children to finish
}
exit 0;