Biblio::ILL::ISO is the base Perl class for the various ISO 10161 interlibrary loan message types (eg: Biblio::ILL::ISO::Request and Biblio::ILL::ISO::Answer).
It knows how to handle all (most?) of the ISO 10161 ASN.1 types (eg: ILLString and ClientId) that make up these messages.
It knows how to do the ASN.1 encoding (from an existing message-type instance) and decoding (from an encoded message into a message class instance (eg: a Biblio::ILL::ISO::Request)).
Treat this class as if it were completely virtual - a program should never instantiate Biblio::ILL::ISO::ISO, but rather the various derived classes (eg: Biblio::ILL:ISO::Request).