my $ssh2 = Net::SSH2->new() ;
$ssh2->connect(HOST) or $ssh2->die_with_error ;
$ssh2->check_hostkey(tofu => HOSTKEY) 
    or $ssh2->die_with_error ;
$ssh2->auth_publickey(USER, PUBLICKEY, PRIVATEKEY)
    or $ssh2->die_with_error ;
$ssh2->auth_ok() ;