From what I've gleaned from perlmonks, if faced with a similar question, I would:

• Pay close attention to Class::DBI. You can set it up to load the "bulk" of a user at once, but leave larger, less-often-used columns out until you need them.
• Tune my database to do the caching for me. Databases are good at this. Well, if they aren't, they should be (try a different database). And, at the very least, they're probably better than I am. Especially once I start scaling. A small, single-machine server is one thing. But if it grows such that a single machine is used solely for the db, and then there are multiple webservers, having the cache done by the db will probably save you time (both in development and at runtime). And then, if you grow further such that you need multiple machines to act as your server, again, I'm going to guess that the database cache will be even more effective for this. Also, the database can take care of concurrency issues, rather than having to worry about stale data sitting in your local cache.

But that's just me. I worry about the unlikely issues of scalability because I'm a dreamer ;-) (That, and if I got a job where it mattered, I'd already have a bit of experience thinking about it.)