I completely removed my lib, sitelib and vendorlib dirs and reinstalled everything from scratch and now Devel::Declare is installed.
in reply to Re^3: Devel::Declare for cygwin; "undefined reference to '_hook_op_check'"
in thread Devel::Declare for cygwin; "undefined reference to '_hook_op_check'"
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!)