I'm having this same problem (yes, seven years later) and this is the only post I've found that matches.

As part of some trial and error I was doing to try to understand what the code (I'm not very good at perl syntax), I tried changing the name of the auth function in SMTP_auth. I was surprised to get an error saying perl could not find the function in Net::SMTP::_SSL. So I changed the ISA in Net::SMTP::_SSL from Net::SMTP to Net::SMTP_auth and AUTH LOGIN worked! I'm not sure how a call to Net::SMTP_auth->auth() ended up getting routed through Net::SMTP::_SSL but it's clear that doing so would be a problem since the auth in Net::SMTP::_SLL has a different number of parameters from the one in Net::SMTP_auth.

So, while I don't have a fix for the problem (other than to hack the ISA in Net::SMTP::_SSL), I'm hoping someone smarter than me can use what I've found to make a proper fix.