# Don't use this - DOESN'T WORK
open my $to_child, ">&=6" or die ">&=6: $!";
open my $from_child, "<&=7" or die "<&=7: $!";
die "fork:$!" if not defined(my $pid = fork);
if ($pid) {
    print $to_child "data for child\n";
    my $ret = <$from_child>;
} else {
    exec { "sub-program" } "sub-program", @args;
}