I think you mean "BLOB", and this is hardly a Perl question.
I'd suggest that you consult your Oracle documentation, in particular the UPDATE command.

You probably want something like:

UPDATE foo SET bar = bar + baz WHERE bar = 'quux';
[download]

Or maybe you need to use CONCAT - I'm not sure. Anyway, this is not the forum for this type of question.

Update: - well, it turns out there is a CLOB datatype after all. My mistake, and my apologies. And thanks gellyfish for setting me straight :)