I completely removed my lib, sitelib and vendorlib dirs and reinstalled everything from scratch and now Devel::Declare is installed.

Either the cpanp uninstall wasn't working, or there was a different incompatible library involved.

Many many thanks for your help - I'm now looking forward to getting MooseX::Declare and trying it out (not too late to restart the project I started yesterday, but I have a lot of other libraries to reinstall now!)