Well, getting this kind of devices to work often involves a considerable amount of trial an error until you find the exact combination of options.

Try requesting a tty on the remote side:

my ($output, $errput) = $kssh->capture2({tty => 1, stdin_data => $esca
+pe_seq}, "vsp");
[download]

Or adding some carrier returns before the escape sequence.

Maybe the device will not accept the sequence until it has send some prompt. In that case, you will like to use Expect, or just use open2 to read the data and send the escape sequence when it is done.

Try also with a timeout and setting the kill_ssh_on_timeout flag.