if the purpose of the module is to expose an OS/platform-specific interface to perl users, then it belongs in the ^Platform:: namespace,

whereas if it implements a generic concept with the internals tailored to work on a certain OS/platform, then it belongs in the generic namespaces with the platform tacked on the end?

I'd say "Yes, generally" to the first part of your question. But I can't think of any precedent for the latter part off the top of my head. A cpan search for "linux" or some such might find some instances of that pattern. Even that wouldn't mean that the convention is blessed by the PAUSE cabal, however.