From KDE 4.0.0 to KDE 4.2.x a call to find_package(KDE4) failed if Perl could not be found. This was an internal, undocumented implementation detail and not necessary, since building software for KDE4 doesn't actually require Perl in general. With KDE 4.3 this has been changed. Now Perl is still searched, but if it is not found, the CMake-run doesn't fail, it just prints a warning message. This way one dependency less is necessary to build software for KDE4. Perl is still required to build kdelibs.