sub untaint_directory {
    # restrict directory names to a short list of accessible locations
    my $var = $_[0];
    my @allowed_locations = qw( political_babble 
                                funny_animal_stories 
                                down_with_starwars 
                                guests 
                                tests 
                                public );
    my $dir;
    my $ok = 0;
    if ( ($dir) = ($var =~ m/^(\w+)$/ )) { 
      foreach my $loc (@allowed_locations) { 
        if ($dir eq $loc) { 
           $ok = 1;
           last;
        }
      }
    } 
    unless ($ok) { 
      die("Not an allowed directory: $dir");
    }
    return $dir;
}