Hmmm ... well ... it's not as easy as you think. It would mean that perl and mongo would share the same or similar associative array implementations. Even if they did, then your approach would never work 100% since perl does not even promise the key order will be the same in different processes.
Did you mean "String *output* from find"? If so, you're probably better of converting the JSON returned from the find into a true JS object and manipulating it that way instead of doing any string manipulation on the results.