This module, when inherited by a package, allows that package's class to define
attribute handler subroutines for specific attributes. Variables and
subroutines subsequently defined in that package, or in packages derived from
that package may be given attributes with the same names as the attribute
handler subroutines, which will then be called in one of the compilation phases
(i.e. in a BEGIN, CHECK, INIT, run-time, or END block).

WWW: http://search.cpan.org/dist/Attribute-Handlers-Prospective/