Edit: I'm smoking crack. Sorry saskaqueer. Old thread is shown below ...

I think you have it backwards, the original poster asked for a way to convert IP addresses to hostnames, not hostnames to IP addresses.

Here's some example code using Net::DNS:

use Net::DNS;
my @domains = qw/google.com perl.com perlmonks.org cpan.org/;
my $res   = Net::DNS::Resolver->new;
foreach my $domain( @domains ) {
   my $query = $res->search($domain);

   if ($query) {
      foreach my $rr ($query->answer) {
         next unless $rr->type eq "A";
         print $rr->address, "\n";
      }
   } else {
      warn "query failed: ", $res->errorstring, "\n";
   }
}
[download]

Be forewarned: Net::DNS is a bit slow, but it does get the job done. Printing a CSV list of the domains and their associated IP addresses is an exercise left to the reader.