Net::SMTP just submits mail to an SMTP server. If you don't run your own, and don't/can't use your ISP's, you'll need to:

• Correctly format outgoing mail according to RFC822;
• Figure out which destination SMTP server to contact for each recipient, based on the MX record for the recipient's domain (you could use Net::DNS for this purpose);
• Queue mail that is deferred when a non fatal error occurs, such as a temporary DNS failure, or an unreachable SMTP server;
• Generate a bounce message when a fatal error occurs, such as an unknown recipient;

All in all this is a lot of work, and difficult to get right. It's much easier to install a MTA on your own machine, open source packages are available for popular platforms.