From the source code it appears that Exception::Class inherits from Exception::Class::Base. The documentation for Exception::Class::Base catalogues a number of class and/or object methods, among which are these:
Returns the description for the given Exception::Class::Base subclass. ... This is also an object method.
Returns the error/message associated with the exception.
So these accessors are, as it were, “built-in” to any class derived from Exception::Class.