Have you checked whether Dancer2 is part of the problem at all? I would try to reproduce the problem with Starman and DBI alone.

A most likely culprit is fork and DBI not playing well (enough) together.

Maybe (maybe) you can reset the database connection when a child is forked off in Starman. I don't know if/how Dancer gets in the way here, so first I would try to get Starman and DBI working and then see if/how Dancer2::DBI needs further changes.