My strange problems with dbish (Command 'quit' not recognized) originated from old dbish script which existed in /usr/local/bin/ which was in path before new one. Since it had 2001 timestamp, I guess it was left-over from times when DBI::Shell wasn't separate distribution.

While this is not exactly same as your problem, i think that hint with which dbish to find out exactly which script are you using helps a lot, so I added it for benefit of monks that find this node with search.
At one point in time I also had similar problem with cpan (shell).