Well, perrin did get me over the 2 GiB hump with the suggestion that I use BerkeleyDB. Now, unfortunately, it complains "Out of memory!" after building about 20 GiB of .db file. (Much larger than I originally anticipated).

So I suspect I'd best go with merlyn's suggestion of using DBD::SQLite, even if SQL does make me itch. Thanks for all your suggestions.