File::Find is a core module, so it is for sure in your installation.
It would be nice if that were true. Unfortunately, some (many?) vendors choose to provide the perl binary separately from the core modules. This means that it is not only possible to have perl installed on a system without the core modules but in some cases (yes, I'm looking at you, RedHat) it is the common scenario. Thankfully those core modules are usually available elsewhere in the distribution and it is usually a simple matter of using the vendor's package manager to install them.
This point has been made a few times before. See for example, Re: Why not include version.pm