my $ssh_debug = 0;  # verbose logging: 0-3
...
    my $session_log_name = "ssh_$host.log";
    open( my $session_log_fh, ">", $session_log_name )
        or die "Failed to create logfile $session_log_name: $!";
    binmode( $session_log_fh, ":unix" );  # unbuffer log file
...
    $ssh_session{$host} = Net::OpenSSH->new(
        # host
        $host,
        # automatically add new host keys to the user known hosts files
        # enable verbosity, if enabled
        master_opts => ($ssh_debug)
        ? [ -o => "StrictHostKeyChecking=no", "-" . "v" x $ssh_debug ]
        : [ -o => "StrictHostKeyChecking=no" ],
        # authentication
        user             => $username,
        password         => $password,
        # logging
        master_stderr_fh => $session_log_fh,
        # connection parameters
        timeout          => $loginTO,
        port             => $ssh_port,
        async            => 1
    );
}