Did you read the documentation for this module? Rather than guessing about what's happening (incidentally, your original script works just fine for me), try turning on the debugging option - just as the docs recommend. Since you're having problems, you should also specify the lookup type, as shown in the docs, instead of hoping for a reasonable default to be assigned.

#!/usr/bin/perl -w
use strict;
use Net::Nslookup;

$Net::Nslookup::DEBUG++;
$Net::Nslookup::DEBUG_NET_DNS++;
print "IP (A record): " . nslookup(host => "yahoo.com", type => 'A');
[download]

Do note that most types of lookups actually return a list rather than a simple scalar. Again, the docs are pretty clear on the issue.

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells