Params::Validate is broken. (I read its code; both the pure Perl and XS code have the same bug.) It thinks that UNIVERSAL::isa is a function. (It's not. It's a method.)
Which version of Test::MockObject::Extends are you using? Newer versions use UNIVERSAL::isa which tries to work around this problem and attempts to reveal which code is broken, buggy, and wrong.
(Of course, if you're using the XS version of Params::Validate, you'll never see the error message because UNIVERSAL::isa doesn't have hooks that deep.)