my (%dhash);

    # Lines assumed to be in @data, adapted from OP's code.
    open(DATA, $resolution) 
      or die("Can't open $resolution for input: $!\n");
    while (my $line = <DATA>) 
    {
        my @parts = split(/&/, $line);
        if (exists($dhash{$parts[1]}))
        {
            $dhash{$parts[1]} += $parts[0];
        }
        else {
            $dhash{$parts[1]} = 1;
        }
    }
    close(DATA);

    # Sample dump routine.
    open(DATA, '> ' . $resolution) 
      or die("Can't open $resolution for output: $!\n");
        foreach my $k (sort(keys(%dhash)))
        {
            printf DATA "%d&%s\n", $dhash{$k}, $k;
        }
    close(DATA);