Net::SMTPS>>> Net::SMTPS(0.04)
Net::SMTPS>>> IO::Socket::INET6(2.72)
...
Net::SMTPS>>> Net::SMTP(3.06)
Net::SMTPS>>> Net::Cmd(3.06)
Net::SMTPS>>> IO::Socket::IP(0.37)
This looks like a bad combination for me: Net::SMTPS has both IO::Socket::INET6 as its direct subclass and IO::Socket::IP as its indirect subclass. The first comes because Net::SMTPS uses IO::Socket::INET6 if available on the system and the latter comes from the IPv6 support builtin into the newer Net::SMTP which you have probably explicitly installed since 5.16 does not come with this newer version by default. And I imagine that this mix of two different IO::Socket modules for the same connection results in the problem.
I would recommend instead to use either Net::SMTP directly which already supports SSL in the version you have installed or use the latest versions of both Net::SMTP and Email::Sender::Transport::SMTP (not SMTPS!) which combined offer the familiar interface you use and also offer SSL.
