I'm not sure I understand your question either, but it sounds like you want to put multiple packages into a single .pm file. You can do this with

package A;

# some code here

package B;

# more code here

package C;

# and still more code
[download]

Save that as Foo.pm and then use Foo; will load packages A, B, and C (and not package Foo, since it's not defined). Packages normally have the same name as the .pm file that contain them and have a one-to-one relationship, but that's by convention (and a very good convention!), not because the language requires it. You can put many packages into a single file or split a single package across many files. But remember that just because you can doesn't necessarily mean you should.

Given the description of what you want to accomplish, you may want to take a look at how the DBI and DBD modules are structured and call each other, since they have a similar design where you just use DBI, then it loads up whichever DBD module(s) it needs behind the scenes without needlessly combining them into a single source file.