UVM_REG backdoor access

Hi , 

     I am trying to implement register backdoor access with user defined register backdoor by extending uvm_reg_backdoor.

        class peri_reg_backdoor extends uvm_reg_backdoor

            virtual task write(uvm_reg_item rw);

      $root.top.DUT.reg = 8'h41;

            endtask  

        endclass 

      when i  am compile this code it is showing error

      illegal location for a hierarchical name (in a package). 

      I want to know what is the overcome for this issue in IUS.

Thanks in advance

sagar