Filesystem:: does not seem to be an established namespace, but Filesys:: is. ZFS:: is also not a top-level namespace, and setting up a new one is often discouraged unless your module is "important" enough - usually though there's an existing namespace that the module fits into. You should probably use Sys::Filesystem::ZFS only if you plan on integrating with Sys::Filesystem. So, maybe Filesys::ZFS? Or something else in the Sys:: namespace. You should probably ask around to get as much input as possible.

See also PAUSE: On The Naming of Modules as well as perlmodstyle. This is also a good question for the perl.module-authors or perl.modules mailing lists.