One way that is useful--especially if you have a local network connected to the net via a NAT box and your external IP is different to your local ip--is to parse it out of a request made to one of the zillions of "What is my IP" pages spread around the net. The address shown below is just the first one google turned up when I searched.

(It would probably not be freindly of us if we all hit this one site. If you want to try this, feed the quoted string above into google, next page 2 or 3 times and then pick one. You might need to tweak the regex a bit.)

#! perl -sw
use strict;
use LWP::UserAgent;

my $ua = LWP::UserAgent->new;
my $request = HTTP::Request->new('GET', 'http://www.internet-help.net/
+cgi-bin/ip-address.pl');
my $response = $ua->request($request);
print $response->{_content} =~ m/(\d+\.\d+\.\d+\.\d+)/;
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