I think this is what you might be looking for:

#! /usr/bin/perl

use strict;
use Data::Dumper;

my (%routers);

#Load Router list
while(<DATA>){
  last if( m/##GROUPS/ );
  chomp;
  my($name, $ip) = (split(/ /, $_))[1,3];
  $routers{$name} = $ip;
}

my $group = 'group2'; #Get group to load here
my (%loaded_routers);

#Load requested group
while(<DATA>){
  chomp;
  s/ //g; #Clear spaces
  my($groupname, $data) = split(/=/, $_);
  if($groupname eq $group){
    foreach my $n(split(/\,/,$data)){
      $loaded_routers{$n} = $routers{$n};
    }
    last;
  }
}

print Dumper \%loaded_routers;

__DATA__
router router1 = 192.168.1.2
router router2 = 192.168.1.3
router router3 = 192.168.1.4
router router4 = 192.168.1.5
router router5 = 192.168.1.6
##GROUPS
group1 = router1,router5
group2 = router1, router2, router3
group3 = router2, router3, router4
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The Output:

$VAR1 = {
          'router1' => '192.168.1.2',
          'router3' => '192.168.1.4',
          'router2' => '192.168.1.3'
        };
[download]

Update: AppConfig looks like a viable option as well. The above will work on the sample data that you gave, but AppConfig is much more robust, and would probably be better it the data gets anymore complicated.