my $msg = "msg\n";

foreach my $w (IO::Select->new($wh)->can_write()) {
    my $rv = syswrite($w, $msg, length($msg));

    if (not defined $rv) {
        die("Unable to send data to child\n");
    }

    if (not $rv) {
        warn("Child is no longer accepting input\n");
        $w_sel->remove($w);
        next;
    }

    if ($rv != length($msg)) {
        warn("Pipe is full. We'd normally call select again\n");
        next;
    }

    print "Successfully sent message to the child\n";
}