Taking a guess at what you are trying to do .. this code may help:

#!/usr/bin/perl
use strict;
use warnings;

local $/="}"; # So we can slurp an entire service in one read
my $host_txt= "host_name";
my $check = "check_nrpe";
my $con = "contact_groups";

while (my $list = <DATA>){
    next if $list=~/PING/; # UPDATE: O.P. Added this sample after I po
+sted
    next unless $list=~ m/$host_txt\s+(\w+).+$check.+$con\s+(\w+)/so;
    print "GROUP NAME: $2; for host $1\n";
}

__DATA__
define service {
       host_name                       KT
       service_description             Check SSH
       check_command                   check_ssh
       max_check_attempts              1
       check_interval                  1
       retry_interval                  2
       check_period                    24x7
       check_freshness                 1
       contact_groups                  admins
       notification_interval           60
       notification_period             24x7
       notifications_enabled           1
       register                        1
 }
 
 define service {
      host_name                       KT
      service_description             Controller Load
      check_command                   check_nrpe!check_load
      max_check_attempts              1
      check_interval                  1
      retry_interval                  2
      check_period                    24x7
      check_freshness                 1
      contact_groups                  admins
      notification_interval           60
      notification_period             24x7
      notifications_enabled           1
      register                        1
 }
[download]

OUTPUT:

GROUP NAME: admins; for host KT
[download]