If your web front-end is triggering writes to the DBM file, and if the quantity of additions/updates is significant, then the problem may be in the DBM module. You could try benchmarking just that part of the application, with or without multiple threads but making sure to simulate a reasonably heavy load of data to be absorbed. (E.g. I know that GDBM really crawls once you start adding data beyond a certain threshold, I think because it has to re-write its entire index at intervals.)

But on the other hand, maybe moving to an RDBMS needn't be so far off as you seem to think -- MySql won't be that hard to install, and getting it working within your current perl/web framework might be easier than you expect. That's worth looking at, seriously.