I'm not an SMTP expert by any means, but perldoc Net::SMTP says:

       auth ( USERNAME, PASSWORD )
           Attempt SASL authentication.  At this time only the
           PLAIN mechanism is supported.

           At some point in the future support for using
           Authen::SASL will be added

I'm guessing that's probably what you're looking for?

The code would look something like:

use Net::SMTP;

my $smtp=Net::SMTP->new('picky.server.com') or die "Can't connect to s
+mtp server";
$smtp->auth('user','passwd') or die "Can't authenticate with smtp serv
+er";
# rest of smtp session goes here, I guess
[download]

You'll have to dig a bit through the SMTP sources, I'm not sure if auth really returns a false value on all failure paths, but I think so.
--
Mike