Here is what i am doing I am sorting and using the index instead of actually writing it as a value in array

my $hash;
foreach my $csn (keys %$datastore) {
  push @{$hash->{$datastore->{$csn}->[3]}}, $datastore->{$csn}->[0];
}

foreach  (keys %$hash) {
    my @sorted = sort { $a <=> $b } @{$hash->{$_}};
    $hash->{$_} = \@sorted;
}

foreach  (keys %$datastore) {
    $datastore->{$_} = [$datastore->{$_}->[1],$datastore->{$_}->[2],$d
+atastore->{$_}->[3], indexof($hash->{$datastore->{$_}->[3]}, $datasto
+re->{$_}->[0])];
}


sub indexof{
    my ($array_ref, $search_for)  = (shift, shift);
    my @array = @$array_ref;
    my( $index )= grep { $array[$_] eq $search_for } 0..$#array;
    return ($index + 1);
}
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Created an other hash with each unique d as key and value is a array ref with sorted a's and then use the index

Sorting the elements of the array wasn't in your spec. Are you suppose to be doing that?