Net::Domain is core:

use strict;
use warnings;
use Net::Domain qw( hostfqdn );

my $host = hostfqdn();
print "$host\n";
[download]

Tested under Solaris, Perl 5.8.8

Update: Obviously this is for the current host, sorry if this doesn't actually answer the question. I'll never post another node while waiting for a hospital appointment to start.