I discovered that it was the source of all of my errors

The errors ("Premature end of script headers") are logged because the warnings are being printed before your HTTP headers, because while you took my advice to wrap the headers in a BEGIN block, you skipped the part of my advice where I explained that the BEGIN block might need to go before certain modules were even used. But getting that right is difficult, which is why I also suggested use CGI::Carp qw(fatalsToBrowser); (because that might help with your debug process).

The warnings ("wide character") are because your script didn't set up the appropriate binmode/open-mode for all the various outputs that are used -- and other monks have given you better advice than I could on that, including Test::More::UTF8 , which I had never heard of, but will definitely keep in my arsenal going forward.

I had planned to use it for the module testing

I will admit that I haven't tested a CGI script, per se. But using Test::More inside the script that's generating the response to the browser seems weird to me. Normally tests are run from the command line (not on your live webserver), and the test script will call the various functions from the modules you wrote that your CGI script is calling. And, if you end up with a lot of logic/etc inside your CGI script that needs testing, you could even have a test that runs your CGI script (CGI can be run from the command line, without the involvement of the webserver -- even the old CGI.pm documentation explained how to do that). Or you could even have an HTTP client inside your test script, which would connect to the webserver to test the endpoints of your CGI (testing on your live server is probably not the best either, but you could have your test suite launch a private webserver instance on your test machine, without it being on your final webhost yet).

But Test::More and the TAP protocol put some of the output (like the test name and ok/not-ok) to STDOUT, but also puts information (like the failure diagnostics) to STDERR -- and trying to properly handle the TAP output inside the CGI environment to generate a valid webpage seems tricky, at best, and I am convinced that (not a problem in Test::More itself) is the cause of your headaches.