Are you running this on Windows? According to the documentation:

On Win32 systems the default setting is equivalent to:

MIME::Lite->send("smtp");
[download]

The assumption is that on Win32 your site/lib/Net/libnet.cfg file will be preconfigured to use the appropriate SMTP server. See below for configuring for authentication.

If you're on Windows, is your libnet.cfg correctly configured? If not, there's your problem.

If you're not on Windows, 'smtp' is not the right option for MIME::Lite->send