As Fletch said, a namespace is not the same as a lexical scope...

This is important to keep in mind if you merge several modules (.pm files) into one file — for easy distribution, or whatever. Forgetting to put extra braces around each original file / package can introduce nasty subtle bugs... (in particular if the modules are coded sloppily, with lots of global vars, no strictures, etc.).

The behavior is mentioned in the package docs, btw:

(...) A package statement affects only dynamic variables--including those you've used local on--but not lexical variables, which are created with my.