Many thanks, Slaven.
The problem actually seems to be somewhere deep inside Test::Effects
(or possibly even deeper inside Test::Builder).
For the moment I've simply recoded the tests to work around the issue.
The module itself appears to be working fine now.
I've uploaded new releases of Test::Effects and Lexical::Failure, both
of which updates are required to overcome the problem under 5.19+.
All the best,
Damian