couch_peruser is a CouchDB daemon that ensures that a private per-user database exists for each document in _users. These databases are writable only by the corresponding user. Databases are in the form:
userdb-{hex encoded username}
For example, the user bob will have a database named userdb-626f62.
The reason for hex encoding is that CouchDB usernames have no restrictions, but CouchDB databases have restrictions. Hex encoding the UTF-8 representation of the username is a transformation that's easy to correctly implement in just about any language, especially JavaScript and Erlang. Other encodings would be possible, but would require additional client and server-side code to support that encoding. This is the simplest scheme that is obviously correct.