my $hostname =
 gethostbyaddr(inet_aton($data[$ip_address_pos]), AF_INET)
         or "Can't resolve $data[$ip_address_pos]: $!\n";

##</code><code>##

use Data::Validate::Domain qw(is_hostname);

if ( $data[$ip_address_pos] eq ""
     && $data[$name_pos] ne ""
     && is_hostname($data[$name_pos])) {
    my $address = inet_ntoa(inet_aton($data[$name_pos]))
                   || "Error: Can't resolve.";
    splice @data, ($ip_address_pos), 1, $address;
}