You might want to take a look at File::CounterFile by Gisle Aas which keeps file-based counters like yours. It uses file locking to handle multiple processes trying to modify the file at the same time, which will be an issue for you because you could have multiple simultaneous HTTP requests going on.

It's very easy to use. This will open the file, increment the value, and return you the updated value:

use File::CounterFile;
$c = new File::CounterFile $data_file, $initial_value;
$raw_data = $c->inc;
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