use Parent qw(function1) # function 1 from Parent
or
use ChildX qw(function1 function2) # function1 from Parent, function2 from ChildX 
or
use ChildY qw(function1) # function1 from ChildY 
or
use GrandchildYX qw(function1 function2) # function1 from ChildY, function2 from GrandchildYX