We located the RPMs for the earlier versions and installed locally. If you have the RPMs on your local filesystem then you can run a yum downgrade command specifying the rpm names as parameters. All four RPMs need to be downgraded as part of the same command or it'll fail on the dependency checks.
You can also add the module names in an exclude line to your yum.conf file if you want to prevent further updates to them (only do this after the downgrade).
@Justin, Casey, Danny:
Thank you all for this.
For those who also have the same issue and struggling to find the steps, here's what I did following the above suggestions / information:
0/ You can verify, if you want, what's installed with
sudo yum --showduplicates list *libapreq2
1/ Download the 2.16-1-1.el7 package from https://koji.fedoraproject.org/koji/buildinfo?buildID=1747448
(I have 2 relevant packages libapreq2.x86_64 and perl-libapreq2.x86_64 so I downloaded those two)
2/ Downgrade using the two packages you just downloaded
sudo rpm -Uvh --oldpackage libapreq2-2.16-1.el7.x86_64.rpm
sudo rpm -Uvh --oldpackage perl-libapreq2-2.16-1.el7.x86_64.rpm
Can verify with the same command sudo yum --showduplicates list *libapreq2
Hope that helps someone.
It's worth noting that version 2.18 of WeBWorK no longer uses Apache/mod_perl, and thus no longer needs Apache::Request.
Since 2.18 is built on Mojolicious it will be much easier to run on current linux distributions.