Internal Program Version: Safe::Isa  1.000010.5.42.0

Since perl allows us to provide a subroutine reference or a method name to
the  operator when used as a method call, and a subroutine does not
require the invocant to actually be an object, we can create safe versions
of isa, can and friends by using a subroutine reference that only tries to
call the method if it is used on an object

http://search.cpan.org/dist/Safe-Isa