`hostname`
program. While sometimes expedient, this isn't very portable. It's one of
those tradeoffs of convenience versus portability.
The Sys::Hostname module (part of the standard perl distribution) will give you the hostname after which you can find out the
IP address (assuming you have working
DNS) with a gethostbyname call.
use Socket;
use Sys::Hostname;
my $host = hostname();
my $addr = inet_ntoa(scalar(gethostbyname($name)) || 'localhost');
Probably the simplest way to learn your DNS domain name is to grok it out of /etc/resolv.conf, at least under Unix. Of course, this assumes several things about your resolv.conf configuration, including that it exists.
(We still need a good DNS domain name-learning method for non-Unix systems.)