Try installing a non-threaded Perl and set Apache to use that instead of the system Perl.

The instructions followed (https://github.com/eanders/CUFTS-Install-Notes/blob/master/CUFTS.mediawiki) mention mod_perl. So your suggestion should probably be changed to:

Try installing a non-threaded Perl and rebuild Apache to use that instead of the system Perl.

which might even be a major pain when Apache was obtained via "apt-get install ... apache2 ..." along with a ton of other packages, some of which come pre-configured to use the instance of Apache provided by apt-get.

It sounds like you are using a Perl which was built with thread support.

As I noted elsewhere, the problem appears to be that Perl threads are actually being created (presumably by Apache). Just having a Perl that supports the creating of threads should not be enough to cause the reported problems (as near as I can tell).