I typically do this:
package Runnable;
sub do_something {
...
}
__PACKAGE__->do_something unless caller;
1;
Then I can execute the module itself as if it were a script. This is great for command-line tools that need to use inheritance or other OO-ish stuff. For a current project, I have my entire Test::Class-based test-suite implemented this way, so the test classes can be run as if they were .t files, but they inherit parent class tests.
Update: Here is a node that discusses this technique in more detail: How a script becomes a module