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. So if you use Perl in your project, check properly for it:
find_package(Perl REQUIRED) # in case you didn't use the REQUIRED keyword: if (PERL_FOUND) ...
This option is only available under Windows, by default it is off. It enables support for generating uac manifest files (or something like that).