I can think of several reasons why not:
in reply to Re: make duplicate JSON keys an error
in thread make duplicate JSON keys an error
JSON::PP apparently is an OO class, but it is not well-factored and compartmentalised in the sense that it is possible to sub-class it to achieve what I want. It has lots of internal global state expressed as upper-scoped variables, like parsers traditionally do, and even refers to the original package name a lot, even complicated by the fact that it treats the package name specially to work together with the optional ::XS variant. Just renaming the package and inserting the check like you said is not enough to make it work.
Having to keep the modified copy in sync with upstream is annoying.
Copy-pasta code opens another can of worms. So far in the project we only have code written on our own, uniformly licenced. Since we distribute the code, I have to spend time figuring out what is affected by this licence-wise "foreign object", and in the end it might well turn out that we just need three additional lines here and there, but that's not at all what I like to spend my time with as a programmer.
Therefore I would go this direction only as a last resort.