with Exception_Handler;

procedure ex_exhandler is
...
    procedure data_error_hp (err_p : Proc) is
    begin
        err_p.all;
    exception
        when T_IO.data_error =>
            T_IO.put_line(T_IO.Standard_Error, "Incorrectly formatted data.");
            T_IO.skip_line;
    end data_error_hp;
-- Error here                                        ↓
    data_error_h : constant Handler := data_error_hp'Access;
begin
end ex_exhandler;


package exception_handler is

    type Proc is access procedure;

    type Handler is access procedure(err_p : in Proc);

...

end exception_handler;