=item Core modules
-Modules shipped as part of the Perl core live in the F<lib/> and F<ext/>
-subdirectories: F<lib/> is for the pure-Perl modules, and F<ext/>
-contains the core XS modules.
+Modules shipped as part of the Perl core live in various subdirectories, where
+two are dedicated to core-only modules, and two are for the dual-life modules
+which live on CPAN and may be maintained separately with respect to the Perl
+core:
+
+ lib/ is for pure-Perl modules, which exist in the core only.
+
+ ext/ is for XS extensions, and modules with special Makefile.PL requirements, which exist in the core only.
+
+ cpan/ is for dual-life modules, where the CPAN module is canonical (should be patched first).
+
+ dist/ is for dual-life modules, where the blead source is canonical.
=item Tests