I would avoid the use of __DATA__ when coding your module; this would make your module incompatable with mod_perl (see: http://perl.apache.org/dist/mod_perl_traps.html for more info.)
Perhaps it is not your intention to use mod_perl, but my basic philosophy is to write code that is as flexable as possible; better to write it this way in the first place, then have to re-write it later.
I would also suggest writing all modules using h2xs. Not only does this lead to a more consistant programming style, you don't have to worry about use lib statements, nor do you have to worry about access permissions to your module files.