The tests packaged in WARC are run without taint mode during installation testing, but in development I use prove -l && prove -Tl to run the tests both without and with taint checks. I assume that perl runs slightly faster with tainting off, and establishing that the code works for the "lower bar" helps to isolate bugs related to taint checking. So far, my test scripts have only needed a few "untaint" modifications — and those have been exactly the places where code using my library needs to not supply tainted input.

I believe the anonymous monk is right — I was going to ask you to run the tests under the debugger to get a full backtrace for the error, but it seems that Test::Script is simply incompatible with Perl's taint mode. I would suggest looking into using IPC::Run3 to check the script's syntax using perl -c (or perl -Tc to do the check with tainting enforced), and then to run the script and capture stderr.