Do you know that aside from the standard phases "develop", "configure", "build", "test", "runtime", and the standard relationship types "requires", "recommends", "suggests", you can also put arbitrary phase and relationship type in your Perl distribution's dependencies (prerequisites, a.k.a. prereqs) using x_ prefix?
The CPAN Meta Spec v2 (CPAN::Meta::Spec) already allows this, but until fairly recently when using Dist::Zilla you can't produce those custom phase/relationship into your distribution's META.json because CPAN::Meta::Prereqs drops all but the standard phases/relationships. But since version 2.150006, thanks to Karen Etheridge (ETHER) (commit link) they are no longer dropped.
What would one use those custom phase/relationship for? So far, Kent Fredric (KENTNL) is using x_examples custom phase to specify prerequisites for examples (which normally will not be installed by your typical CPAN client). Karen plans to "use some new prereq categories for travis to make use of when deciding what prereqs to install for different tasks."
I myself am using x_spec relationship (in the develop phase) to specify that a distribution follows some specification, for example all my distributions which contain some Rinci metadata is peppered with this. Another one is x_embed relationship. This is when I embed a module source code into another module using fatpacking technique using Module::FatPack, to reduce dependencies. The embedding distribution no longer needs to specify a runtime requires dependency for the embedded module, but I still would like to record the fact that the module is being embedded, so I can know when I want to update the embedded source sometime later.
The possibilities are endless.
A somewhat related issue, which is broader, more complex, and hasn't been solved, is specifying dependencies to things that are not Perl modules, e.g. an OS package, a certain program, a certain C library. (References: #79, #82).