If the option StrictHostKeyChecking=no is passed to OpenSSH ssh client and no key exists on the known_hosts file for the target host, it will accept as good (and remembered for future connections) the first one received from the remote host.

Though, I don't see how extra options for ssh can be passed through Net::SCP::Expect.

Anyway, you can use Net::OpenSSH instead:

$ssh = Net::OpenSSH->new($host,
                         user => $user, password => $password,
                         master_opts => [-o => 'StrictHostKeyChecking=
+no']);

$ssh->scp_put(...);
$ssh->scp_get(...);
[download]

Note that disabling strict host key checking has some security implications that you should be aware of.