One of the tests in cperl-mode-tests.el starts an external Emacs process
under timeout to check for an endless loop.  It turns out that the
timeout of 1 sec is rather short for slower machines.  Thanks to recent
ERT which now reports the run time in batch tests, I saw that it takes
about 0.7 seconds on my machine, so I increased the timeout to 2
seconds.

I also flagged the test as "expensive" and skip it under older Emacs
versions (I run tests for Emacs version 26.1 and above).