The DBI table_info method should be helpful in dredging up what sort of columns your db abstraction layer is producing. The NULLABLE field will tell you whether things written as undef will be returned as empty strings.

That sort of problem is why I'm suspicious of dbi abstractions. I prefer to have a collection of carefully designed db schema which can be selected and plugged into an application. That kind of close coupling between SQL column attributes and perl data is a pain to maintain if you do it ad hoc. A carefully kept library of common column types and tables is not that much easier to maintain, but there is only one of them.

Is your toolkit using CGI.pm? With CGI, what you write as keys %params is returned by the param method without arguments, $q->param.