I take it you want to know how multiple inheritance in Perl works, i.e. how method resolution is done? Quoting the 3rd edition of Programming Perl (p. 322):

2. Next, Perl checks for methods inherited from base classes by looking in all parent packages listed in @classname::ISA for a parent::methname subroutine. The search is left-to-right, recursive, and depth-first. The recursion assures that grandparent classes, great-grandparent classes, great-great-grandparent classes, and so on, are all searched.
3. [...]

Emphasis mine. perlobj also has more on multiple inheritance.