A,B*,item2,,
A,B*item1,,,
A,B*,,,item4
C,D*item1,,,
C,D*,,item3,

##</code><code>##

sub merge {
      open(INFILE, "test.put");
      my %data;
      while (<INFILE>) {
      chomp;
      my ($key,@items) = split(/\*/);
      $data{$key}{$_}++ for @items;
      }

  close INFILE;
  print Dumper(\%data);
}

##</code><code>##

$VAR1 = {
          'C,D' => {
                     ',,item3,' => 1,
                     'item1,,,' => 1
                   },
          'A,B' => {
                     ',,,item4' => 1,
                     ',item2,,' => 1,
                     'item1,,,' => 1
                   }
        };

##</code><code>##

C,D,item1,,item3,
A,B,item1,item2,,item4