But then the code is not usable from a module, no?

A fully qualified global name is always accessable and usable from everywhere. The "problem" I think you are indirectly referring to is when you have a module X that uses globals that exist within the package that other code must then fully qualify using the full package name. For a variable named $X::x thats not so bad, but when the package is Some::Long::Package::Name it starts getting clumsy.

The options are to export the globals in to the consumers package or to use "special globals" (like $a, $b, and $_) or to just use vars in package main. Assuming that you don't have a lot of globals in use in package main (IME a safe assumption) using such vars is IMO a reasonable choice. In other words typing $::Hash isn't so tough. Typing $Some::Long::Package::Name::Hash is.