#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my $curr_grp;
my (%host, %user, %domain);
while (my $line = <DATA>) {
    if ($line =~ /^\s*(?!<\()([\w-]+)(?!=\))/) {
        $curr_grp = $1;
    }
    if ($curr_grp) {
        my @entries;
        push @entries, $1 while $line =~ /\((.*?)\)/g;
        for my $entry (@entries) {
            my ($host, $user, $domain) = split /,/, $entry;
            push @{$host{$curr_grp}}  , $host   if $host  ;
            push @{$user{$curr_grp}}  , $user   if $user  ;
            push @{$domain{$curr_grp}}, $domain if $domain;
        }
    }
}

print Dumper \%host;
print Dumper \%user;
print Dumper \%domain;

__DATA__
nifty-group (host,,zanzibar.org) (host-1,,) (host-1.domain,,) 
(host_234,,) (foo,,) (host-1,phil,) 
(host-1.domain,,) (host_234,,)

other-group (h2,,) (h6,,)