There's nothing strictly speaking which directly ties the package namespace the contents of a module defines with the filename containing it. You could have a file Fred/Ethyl/Mertzomatic.pm which contained a line package Ricky::Lucy::Recardotron; and use Fred::Ethyl::Mertzomatic; would succeed fine so long as it parsed and returned a true value. You can also have code in several different namespaces in the same file if you like (often used to declare subclasses or subsidiary packages which are pulled in by using the top level module).

However use will look and see if the package corresponding to the name it was given has an import method and will call that method if it exists; since your example doesn't define a method dir::mod->import (directly or through inheritance; you do declare, implicitly through inheritance, mod->import though) Exporter's import is never called and so the assignment of \&mod::hello into *main::hello never occurs and you get the undefined sub error at runtime.

That clear things up?

That makes sense. Thanks!

Well, I am not so familiar with the internal Perl actions, but I assume the problem is something like this:

require dir::mod works, as it is translated into a file path dir/mod.pm and that file is found and read. Then it tries to import the exported stuff from the dir::mod namespace. But this is not defined as your package name is mod, and not dir::mod.

If you'd call mod::hello() your script should work.