In an attempt to solve this mystery testing issue, I've minimised the JSON object and put it in a string within the test code:

my $payload_pass = '{ "test": "test payload and signature", "type": "i
+nvoice.paid", "object": "event" }';
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So it is no longer read in from __DATA__.

When CPAN Tester results start to arrive, we'll discover if this solves the sue...

Update
It looks like it might not. More test results are in and it appears to be OS dependent. It is passing on Windows and failing on everything else.