Well, from Mail::Sendmail's documentation it's clear that if @{$Mail::Sendmail::mailcfg{'smtp'}} isn't set, then it tries to send through localhost.

It would appear that you don't have a sendmail (or other MTA) on your port 25.

You can test that by doing

telnet localhost 25

I'm betting you'll get connection refused.

And that means you either have to pick another server that will allow you to send mail through it, or install a mailserver on your localhost.