Then thread-safety isn't such a big deal if you do it like

my $safeIfTheOnlyOne = threads->create('oneAndOnlyMailer', 'argument')
+;


sub oneAndOnlyMailer {
   require IO::Socket::SSL;
   require Mail::IMAPClient ;
   require Net::IMAP::Simple::SSL;
   ...
}
[download]

If you need multiple mailers, then it would pay to make IO::Socket::SSL thread-safe