use strict;

my $file1 = "data1.txt";
my $file2 = "data2.txt";

my $raw_data1 = Slurp_File($file1);
my $raw_data2 = Slurp_File($file2);

my (@sections1) = ($raw_data1 =~ m/(<host .+?\/host>)/sig);
my (@sections2) = ($raw_data2 =~ m/(<host .+?\/host>)/sig);

my %parsed_file;
foreach my $section (@sections2) {
  my ($id,@parsed_data) = Parse_Section($section);
  foreach my $alias (@parsed_data) {
    $parsed_file{$id}{$alias}++;
  }
}

foreach my $section (@sections1) {
  my ($id,@parsed_data) = Parse_Section($section);
  foreach my $alias (@parsed_data) {
    if (!$parsed_file{$id}{$alias}) {
      print "HostID: $id, Host-Alias: $alias was missing from file '$file2'\n";
    }
  }
}

############

sub Slurp_File {
  my $file = shift;
  my $data;
  open(DATA,"<",$file) || die "Unable to open file '$file':  $!\n";
  {
    local $/;
    $data = <DATA>;
  }
  close(DATA);
  return $data;
}

sub Parse_Section {
  my $data = shift;
  my ($id) = ($data =~ m/id=\"(.+?)\"/i);
  my (@alias) = ($data =~ m/host-alias>(.+?)</ig);
  my (@list) = ($id,@alias);
  return @list;
}