In the general case, there is not enough information in a diff file (in isolation) to be able to give such context - because in the general case there is no guarantee that unchanged entries such as the id will appear in the context lines. Because of that there is unlikely to be a general-purpose module available for this.

In your specific case, maybe it is possible - if the JSON is guaranteed to be short enough that the id and timestamp will always appear in the context lines. In that case, a hand-rolled parser for the diff chunks should be easy to construct and use; you may be able to get a leg-up by finding something that already parses diff files (such as the inline "Patch" class in patch, part of PerlPowerTools), but it may well be overkill for your needs.