My concern is that because of the use of threads in such frameworks, keeping a DB handle over time will lead to problems.

The only way to quell or prove the anecdotal theories you have, is to test for yourself. Take the time to set up whatever DB configuration you want, then write an automated test suite to hammer the db code until it either breaks, or is usable beyond what you perceive to be acceptable.

FWIW, I prefer Dancer2, but I digress.