sub get_group {
  my $user_id = shift;
  my @special_groups = get_special_group($user_id); # ex: 'cdrom', 'games'
  my @normal_groups = get_normal_group($user_id); # ex: 'westteam', 'marketing', 'committee'

  return @special_groups, @normal_groups;
}

my(@specail, @normal) = get_group(1002);
print $special[-1]; # 'games'
print $normal[0]; # undefined