[Koha] to cpan or not

In the instructions for Debian Lenny 5.0 (INSTALL.debian-lenny) there is a dire message that was pointed out by developers as: * IMPORTANT: You should only use CPAN for Perl dependencies which are NOT available from the package maintainer. You have been warned! * With repect to installing dependencies, how does one know which Perl dependencies ARE available and when do we rely on CPAN to replace them? Is the "package maintainer" the original Koha download and if so, how do we know what packages are missing?