%host_groups = get_host_groups();
foreach my $group (keys %host_groups) 
{
    print "$group = $group\t";
    foreach (@{$host_groups{$group}}) {
        print "$host = $_\n";
    }
}


sub get_host_groups() {
   ....
   while ($line = <FILE>)
   {
    ....
    $group_name = $hgn[2];
    ....
    $hash{$group_name}= [@hosts];
    ....
    ....
   }
   return (%hash);
}