I see a lot of warnings like
Subroutine main::to_json redefined at (eval 2673)[t/00_required_modules.t:54] line 2.
Prototype mismatch: sub main::to_json ($) vs ($@) at (eval 2673)[t/00_required_modules.t:54] line 2.
As well as JSON related test failures when running test coverage on a large project. Tests all pass if Devel::cover isn't used.
Some code to reproduce the issue would be helpful. And also your perl and Devel::Cover versions.
Thanks very much for the report, but I've not been able to reproduce this. @sanbeg: if this is still a problem please reopen the ticket and provide some more information. Thanks!