I found a better way than the hierarchical clustering.

The code below generates a line for each combination: (a) count of attributes, (b) count of items, (c) attribute list (d) item list.

I pass it through sort -u because there may be duplication.

1 4 round apple,ball,orange,pumpkin
2 2 red,round apple,ball
2 3 plant,round apple,orange,pumpkin
3 1 red,round,toy ball
3 2 fruit,plant,round apple,orange
3 2 orange,plant,round orange,pumpkin
4 1 fruit,orange,plant,round orange
4 1 fruit,plant,red,round apple
4 1 orange,plant,round,vetetable pumpkin
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The lines that trivially report a single item and all its attributes would come into play if you had multiple items with the same attribute set.

The code is below. Basically what happens is that for the item2attr hash, I look at all unique attribute lists across items and then report the items that have this set in common. This comes into its own when you flip the hash and report the same thing, but flip the role of item and attribute. In the end you get a full list.

I hope that you find this useful and that it does what I think it does :)

my $item2attr = {
            apple   => {red=>1,   round=>1,plant=>1,fruit=>1},
            orange  => {orange=>1,round=>1,plant=>1,fruit=>1},
            pumpkin => {orange=>1,round=>1,plant=>1,        vetetable=
+>1},
            ball    => {red=>1,   round=>1,                           
+ toy=>1},
           };

## alternatively in the block below, generate a random data set with
## 500 items and 75 attributes with randomly 2-10 attributes per item
=pod
my $n_items      = 500;
my $n_attributes = 75;
my $min_attr_in_item = 2;
my $max_attr_in_item = 10;
$item2attr = {};
for my $i (1..$n_items) {
  my $item = sprintf("it%03d",$i);
  my $n_attr = $min_attr_in_item + rand(1+$max_attr_in_item-$min_attr_
+in_item) ;
  my @attrs  = sort ((map { sprintf("at%03d",$_) } (sort {rand() <=> r
+and() } (1..75)))[0..$n_attr-1]);
  #printinfo($item,int(@attrs),@attrs);
  map {$item2attr->{$item}{$_} = 1} @attrs;
}
=cut

# list of all items and attributes
my @items = sort keys %$item2attr;
my @attr  = sort(uniq( map { keys %$_ } values %$item2attr));

# flip the hash 
my $attr2item;
for my $attr (@attr) {
  map { $attr2item->{$attr}{$_} = $item2attr->{$_}{$attr}  if $item2at
+tr->{$_}{$attr} } @items;
}

report_sets($item2attr);
report_sets($attr2item,-swap=>1);

sub report_sets {
  my ($hash,%args) = @_;
  my $sets;
  for my $key (keys %$hash) {
    my $set_hash_str = join(",", sort keys %{$hash->{$key}});
    $sets->{$set_hash_str}{$key}++;
  }
  for my $set_hash_str (keys %$sets) {
    my @attr  = split(",",$set_hash_str);
    my @shared_attr = shared_items($hash,@attr);
    if($args{-swap}) {
      printinfo(int(@shared_attr),int(@attr),join(",",@shared_attr),jo
+in(",",@attr));
    } else {
      printinfo(int(@attr),int(@shared_attr),join(",",@attr),join(",",
+@shared_attr));
    }
  }
}

sub shared_items {
  my ($hash,@attr) = @_;
  my @shared_items;
  my @items = keys %$hash;
  for my $item (@items) {
    my $n = grep($hash->{$item}{$_}, @attr);
    push @shared_items, $item if $n == @attr;
  }
  return sort @shared_items;
}
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