Are core modules permissible?

File::Find can be used to traverse a portion of a filesystem and tally things up. It's a rather inefficient solution if you want an entire volume's usage. The utilitydf would be better for information on an entire volume, and can be wrapped in a Perl call. And if you need to tally usage of a portion of a volume du would be useful. But if du and df are off the table, File::Find along with stat are good starting points.

The File::Find module has been bundled with Perl since Perl 5 began. If it is not allowed as part of the solution, then push back on that requirement, because the same logic would also prevent using strict and warnings.