the max length for a base64 coded line required by rfc2045 for MIME encoded data (mails).

No idea if the 60 has any hard technical background though.

Anyway, I don't know if such a character level encoding is the solution to Mike's problem. After all, this is completely unaware of the structure of the data encoded.

Serializer/Formats as Data::Dumper, XML, YAML generate output which can be described by contextfree languages, which probably is no coincidence. Resorting to the pumping lemma the maximal distance between the start/end braces/tags/indentation-changes corresponding to the start/end of an embedded serialized substructure is not limited, which sounds quite incompatible to "chunk" (i.e. fixed-length) to me.