Quoting File::ShareDir (emphasis mine):

Quite often you want or need your Perl module (CPAN or otherwise) to have access to a large amount of read-only data that is stored on the file-system at run-time.

If you want ordinary users to create their own data files, then File::HomeDir is for you. In particular, look at the functions my_dist_data and my_dist_config.

If you want a shared file with r/w access in your module's directory, then the absolute path can be identified by loading the module and inspecting its entry in %INC. Since files in the lib section are usually installed read-only, you'd still need to modify the permissions during installation of your module. From a security perspective, a globally-writeable file will probably cause some nasty issues because it's not only your application which can write the file.