This is a the Dist::Zilla manpage plugin that runs at the
Dist::Zilla::Role::FileGatherer stage, providing a test file (configurable,
defaulting to _xt/author/clean-namespaces.t_).

This test will scan all modules in your distribution and check that their
namespaces are "clean" -- that is, that there are no remaining imported
subroutines from other modules that are now callable as methods at runtime.

You can fix this in your code with the namespace::clean manpage or the
namespace::autoclean manpage.