I would try document the behaviour. Maybe something like:

=head1 Handling of file classification

For every file category as found via C<< ->find_categories >>,
the module provides a function C<< find_FILE_CATEGORY_files >>.
In the below example, the C<< get_cats_files(...) >> function returns
all files in the C<< cats >> category:

    my $cat = find_category_for_file( 'myfile_cats.txt' );
    # cats
    print for get_cats_files('/*');
    # myfile_cats.txt myfile2_cats.txt ...
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Personally, while AUTOLOAD is enticing, why not make the category a real parameter instead? Otherwise, the programmers will have no good way of passing an arbitrary category to your code.