The basics are easy enough. Say you want to make a module named My::Foo. You put a package My::Foo; at the top of the module file to declare what package it is in. After that, you put all your subroutines you want in that file. Modules need to return a true value to indicate that they were loaded successfuly, so you put a 1; at the very end of the file. It is common to put an __END__ declaration after that, but not required.

To access the module, you need to place it in some directory specified in the @INC global array. Since the module is named My::Foo, which uses the :: sperator, we need to place our module in the correct subdirectory under some @INC dir. The :: seperates the directory path, so we place the module under My/Foo.pm (note the .pm extention, which all Perl modules have).

There are a few ways to call your subroutines. For all of them, you have to put a use My::Foo; before you call any of them. Then you could call a subroutine named bar() like this:

My::Foo->bar();   # 'My::Foo' will be the first param passed 
My::Foo::bar();   # doesn't touch the param list
[download]

You can also use the Exporter module to call bar() without specifiying the full module--see the documentation for Exporter.pm.

That's the quick-and-dirty overview, which glosses over a lot of cool stuff.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated