How do I find out my hostname/domainname/IP address?

How do I send mail? Well, I would have linked to perlfaq9 again for that except that it mentions using Mail::Mailer (which I could not even find using ppm3) or Net::SMTP (which I have only had limited success with on Windows). So I'm going to use Mail::Sendmail. After you have installed it, you should edit Mail::Sendmail.pm and change the line to show your actual SMTP server:
    'smtp'    => [ qw( localhost ) ],

Combine everything together and what do we get?

#!/usr/bin/perl -w
use strict;
use Socket;
use Sys::Hostname;
use Mail::Sendmail;
my $from = "foo\@bar.com";
my $to = "bar\@foo.com";
my $subject = "Latest IP address";
my $host = hostname();
my $addr = inet_ntoa(scalar gethostbyname($host || 'localhost'));
my %mail = (To      => $to,
            From    => $from,
            Subject => $subject,
            Message => $addr
           );
sendmail(%mail) or die $Mail::Sendmail::error;
print "OK.\n", $Mail::Sendmail::log;
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Answer: A cross-platform solution!