in reply to
Re^2: flush error in modperl on system commands
in thread flush error in modperl on system commands
One more follow up: removing Test::More solved the problem on my production server (I stress tested it with 10000 consecutive queries) but did not solve the error on my dev server (and yes, both servers exhibit the error when Test::More is included). Nothing else on the dev server is using any Test package that I could find. For the time being, since the production server is working this is satisfactory for me so I am no longer actively seeking a solution, although it is still very curious to me.
I won't call this solved, but for any seekers of perl wisdom with this issue in the future I recommend checking libraries you are including, especially any library from the Test package.