I have had several experiences where my unit-test suite passes, but Devel::Cover fails. I use the testcover target from Module::Build, but experienced the same problem when calling Devel::Cover in the conventional way.
I found this thread about the problem. But I am not complete convinced on how to proceed.
After getting test coverage to 98.6% on Business::OnlinePayment::CashCow I wanted to get high coverage on Date::Holidays aswell, since I was working on an update of this module.
This is the trouble some part
t/is_holiday......ok 11/12# Looks like your test died just after 12.
t/is_holiday......dubious
Test returned status 255 (wstat 65280, 0xff00)
after all the subtests completed successfully