To borrow something from java's convention (I'm not exactly fond of java, but ah well), you can use your organization or name to provide a very reasonable namespace.

Certaintly there could be another FordMotorCompany::Gizmo and there may be another AlanSmithee::Gizmo, but they wouldn't be uploaded to CPAN.

Java handles this as follows: com.ford.foo.bar.absurdly.long.package.namespace.Gizmo. Not too elegant, but the general idea is borrowable, methinks. They are using URL registration to represent ownership. In general, I don't like this, as it implies you have a URL to yourself. Not everyone does.

For the more creative, pick some random nonsensical phrase that anyone would be too embarassed to use on CPAN. Such as ILoveFrisbeesAndMooses::Gizmo :)