#!/usr/bin/perl

# http://perlmonks.org/?node_id=1191400

use strict;
use warnings;

my @array = qw(
  ULTRIX.CW18.72.0.3.IP-HOST1.log
  DEC_DECSTATION.CW180.72.0.3.IP_HOST_AL.log
  DEC_DECSTATION_ADDR.CW180.72.0.3.IP_HOST_al2.log
  FOR_VISITORS23_HOST.HL617.253.1101.2.IP_HOST_hostinfo.log
  FOR_VISITORS24_HOST.HL617.253.1101.2.IP_HOST_hostinfo2.log
  FOR_VISITORS25_HOST.HL617.253.1101.2.IP_HOST_webform3.log
  );

my %hash1;

/^(.+?)\.(.+?)\.IP/ and push $hash1{$2}->@*, $1 for @array;

use Data::Dumper;
print Dumper \%hash1;