CTEST_CUSTOM_PRE_TEST is a variable used in the context of running a ctest dashboard. It should either be set directly in the ctest -S script itself, or in a CTestCustom.cmake file at the top of your build tree.
In either file, an example value might be:
set(CTEST_CUSTOM_PRE_TEST "perl prepareForTesting.pl -with-this -and-that")
It should be a single command line, properly formatted for running on the system you're on. It runs once during a ctest_test call, before all the tests run. Similarly, there is also a CTEST_CUSTOM_POST_TEST variable, that should also be a single command line, but runs after all the tests are done.
Quoting and escaping args with spaces, quotes and backslashes may be challenging ... but maybe you won't need that, either.
I do not know of a real world example of this that I can point you to, but I can read the ctest source code... ;-)