use strict;
use feature qw(say);
use Socket;
use File::Tail;

my $file = File::Tail->new("/some/log/file");
my $seen = {};
my $line;

while (defined($line = $file->read)) {
    if ($line =~ /^DATE (IP) (WHATEVER)/) {
       my $remote_ip   = $1;
       my $whatever    = $2;
       my $remote_host;

       if ($seen->{$remote_ip}) {
           $remote_host = $seen->{$remote_ip};
       else {
           $remote_host = getaddrinfo(inet_pton($remote_ip),AF_INET) || 'none';
           $seen->{$remote_ip} = $remote_host;
       }
       say join "\t", qw/$remote_ip $remote_host $whatever/;
    }
}