I typically do this:

package Runnable;

sub do_something { 
   ...
}

__PACKAGE__->do_something unless caller;

1;
[download]

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