packages are namespace areas so that symbols with similar names do not clash with one another. For example, the symbol &main::first is different from symbol &List::Util::first . Packages are name prefixes to sysbols.

A module is a file of code, or a tree of bytecodes. A module could be precompiled (.pmc), uncompiled (.pm) on disk; or pre-loaded in memory as one unit -- assuming no autosplit.

In summary: packages are about namespaces, and modules are about files. They are different things, like apples and boxes -- until the day when you start placing one kind of apple into one kind of box, and people start thinking that apples and boxes are related. And they are! but still, one in fruit and the other is a type of container.