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, or END block).

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