my $child_pid = fork();
if ( $child_pid ) {
    # parent goes about its business
}
elsif ( defined $child_pid ) {
    # child
    my $pid = open(PID, "|$cmd")
        or die "Can't fork: $!";

    # blah blah

    # this hangs until process_tar.pl finishes.
    close(PID) or die "Can't close: $!";
    exit;
}
else {
    die "Can't fork: $!";
}