In https://rt.perl.org/rt3/Ticket/Display.html?id=105922, we have made the decision to not differentiate between strings that happen to look like vaguely like a package name and strings that don't (so "Foo"->isa("Bar") and "3Foo"->isa("Bar") will do the same thing, aside from the different package names). This breaks t/fail-more.t, which expects isa_ok(42, "Wibble") and such to return different errors. The error message that is generated when this change is made to blead causes the issue reported at https://rt.cpan.org/Ticket/Display.html?id=78204 to start happening in Test::More 0.98 as well, so that error message will need to be adjusted.
I have submitted a patch to adjust the tests at https://rt.cpan.org/Ticket/Display.html?id=79762.
(GitHub doesn’t allow attachments, which is annoying when I already have a diff file.)
This was fixed by 5a26767 coming soon to a stable release near you. (already in 0.98_04, I believe)