So, what's the big idea?
Perl projects have all manner of ways of declaring their dependencies. CPAN releases usually include a file called META.yml or META.json listing their dependencies (though Makefile.PL or Build.PL is also supposed to generate a list of dependencies when it runs; this allows the release to dynamically decide on different dependencies based on the machine it's running on). Non-CPAN projects can declare their CPAN dependencies using cpanfile too.
Once the dependencies are declared, this information is used by CPAN clients, by metacpan.org (to show the list of a release's dependencies), by http://deps.cpantesters.org/ and so on.
However, this only works where you want to declare dependencies on CPAN modules, or on a minimum version of Perl itself.
There's another few important types of dependency - your project may depend on external libraries or binaries, or may depend on a particular platform (e.g. code that only works, or only makes sense to use on Windows).
For external libraries, there is good work going on in the Alien namespace on CPAN. And it seems reasonable to extend this approach to binaries.
For platforms, the solution to this has often been to include code in Makefile.PL to check
$^O looks OK and die otherwise:
die "Windows only!" unless $^O eq "MSWin32";
However, this approach doesn't leave any indication of the required platform in META.yml; it's invisible to services performing automated analysis of CPAN; metacpan.org's dependency listings won't show it.
The Platform namespace is designed as a solution to these issues.
What is in the Platform namespace?
Initially three releases. The first is Platform itself - a documentation-only release explaining the concept.
The next is Platform::Unix. This is a module that doesn't "do" anything, but will only install and only run on Unix-like platforms. So if your Perl project requires a Unix-like OS, just add a dependency on Platform::Unix, and Bob's your uncle. Having Platform::Unix listed in your META.yml and on your metacpan.org page makes it really explicit to your users that your release requires a Unix-like operating system.
The third release is Platform::Windows which is much the same thing for Windows.
Any more planned?
No, but if you have any requests, let me know. I've got a template for building new Platform releases in the Platform repository.