What is tested
There has been a large amount of testing done of the main styles by a commercial project inside Editure.
However the automatic and unit testing of the applications and code has only just begun.
Testing XSL
The XSL is being tested the same way perl modules. This is done mainly by calling scripts to do the test by running the XSL over a known bit of XML data and checking the results produced - very similar to testing a function call.
Testing Perl
The standard perl modules are easy to test with normal perl test framework. The harder modules are the mod_perl handlers (e.g. Auth or Fixup Handlers). These are going to be tested using Apache (same as mod_perl does) and using Mock Objects.