file.txt
--------------
object-group network HOSTNAME_1ST
 network-object host 10.1.1.1

object-group service WEB_TCP tcp
 port-object eq 80
--------------

##</code><code>##


use strict;
use warnings;

my $iphostname;

my $filename = 'file.txt';
open(my $fh, '<:encoding(UTF-8)', $filename)
  or die "Could not open file '$filename' $!";

for $iphostname ($fh) {
  print $iphostname if (/^object-group/ ... /(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/);
}