You can use polling mode on cmd() if you set Blocking to 0

So you could do this:

$s1_cli->cmd(
    Command  => "/path/to/pre-loader.pl arg1 arg2 2>&1",
    Blocking => 0
);
do {
    Time::HiRes::sleep 0.2;
    ($ok, $output) = $s1_cli->cmd_poll;
    print $output if length $output;
} until $ok;
[download]

Or else this:

$s1_cli->cmd(
    Command  => "/path/to/pre-loader.pl arg1 arg2 2>&1",
    Blocking => 0
);
$s1_cli->poll(
    Poll_code => sub {
        $output = ($s1_cli->cmd_poll)[1];
        print $output if length $output
    }
);
[download]