Thanks for the suggestions. This is a situation where it's easy to accidentally create a closure, and rare to have a legitimate use (if there is a legitimate subroutine that uses a closure, it could just be put into a named subroutine, and an anonymous one that's passed to do_transaction could call the named one), so at least a warning on the dev server would be useful. Having used mod_perl for a while, I've seen that accidental closures can be hard to track down, whether they were written by me or someone else on the project.