My apologies. Let me give you more details on this. Basically, I have routine to read a file(test.out) where the file format is:

A,B*,item2,,
A,B*item1,,,
A,B*,,,item4
C,D*item1,,,
C,D*,,item3,
[download]

Then my code is

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

  close INFILE;
  print Dumper(\%data);
}
[download]

The Dumper shows

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

If I want to merge the rows under key (e.g. 'C,D") to reformat and write into file, what should I do then? many thanks

C,D,item1,,item3,
A,B,item1,item2,,item4
[download]