It's a simple non object oriented module.

I would tend to lower-case this variable as stevieb would, but for me the more important consideration is use of the fully-qualified format $Data::GUID::Any::UC (or $Data::GUID::Any::uc) for a variable of this type which is intentionally not exported from its module; this format makes it quite clear what's going on IMHO, i.e., use of a package-global. Lc-first camel-casing is also an option for compound names: $Data::GUID::Any::upperCase.