I don't know how portable this is. I'm on Unix. Also, this code needs checking of system call return values before serious use!

use Sys::Hostname;
use Socket;

my $hostname = gethostbyaddr(gethostbyname(hostname()),AF_INET);
print "$hostname\n";
[download]

Update: The reason I included the calls to gethostby* is in case you want the domainname also. hostname() from Sys::Hostname doesn't always include that.

-- 
dempa