When you define a function, or import one, into a Perl package, it will
naturally also be available as a method. This does not per se cause problems,
but it can complicate subclassing and, for example, plugin classes that are
included via multiple inheritance by loading them as base classes.
The namespace::clean pragma will remove all previously declared or imported
symbols at the end of the current package's compile cycle. Functions called in
the package itself will still be bound by their name, but they won't show up as
methods on your class or instances.
By unimporting via no you can tell namespace::clean to start collecting
functions for the next use namespace::clean; specification.