Every aspect of Perl's operation has a related test, and these test files live in the t/ directory. Tests for individual library and XS modules are slowly being relocated to lib/ and ext/ respectively, but at time of writing, there are over 23,000 separate tests in over 400 test files.
On a related note, functions for debugging Perl itself are to be found in deb.c and dump.c. The distinction is that functions in deb.c are typically accessible from the -D flag on the Perl command line, whereas things in dump.c may need to be used from a source-level debugger.