1: Both ways are valid:

$s1 = Net::OpenSSH->new($host, ...);
$s2 = Net::OpenSSH->new(host => $host, ...);
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2: No, by default Net::OpenSSH let's ssh ask you questions. If you want batch mode you have to state it:

$s = Net::OpenSSH->new($host, batch_mode => 1, ...)
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3: Yes because Net::OpenSSH only works with version 2 of the protocol. SSH v1 is not supported.

4: The connection is closed automatically when the object goes out of scope. You can close it explicitly if you want:

$s->disconnect;
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update: disconnect method is only available in the latest development versions.

Also, if you need to connect using SSH v1, my other module Net::SSH::Any does support it.