You should NOT be use this module. It has not been maintained in 7 years. It is broken in several ways (see the RT bug reports). A successor fork called Net::SMTP::TLS::ButMaintained is available which fixes a number of the bugs in this module. While ButMaintained is better, neither module is very good.
I was hoping this module would be a drop-in replacement for Net::SMTP with TLS support. While testing, I noticed the SMTP behavior was poor (not RFC compliant in several ways) and I fixed a couple of the problems. However, this module is not programmer friendly and in quite a few ways, does not behave like Net::SMTP.
After trying to use this module, I thought to myself, "I would be better of writing myself a thin layer for Net::SMTP that adds STARTTLS support." But before I put paws to keyboard, I searched CPAN again, and found Net::SMTPS, which is exactly what I had envisioned. Consider using it instead.