Part of the reason that Debian needs Synaptic is because the Debian package distribution and installation system is, quite frankly, not CPAN. Among other things, the configuration system built into CPAN.pm is really quite good, and something that Linux distributions (on both sides of the deb/rpm divide) would do well to have a good hard look at. Just for example, selecting CPAN mirrors is a good deal more straightforward, and requires a good deal less up-front knowledge the first time, than adding package sources in Synaptic, to say nothing of the command-line interface to apt. You answer a couple of easy multiple-choice questions about where you are located, and it shows you a list of mirrors, and you pick some of them. I wish it were that easy in Synaptic (or rpmdrake)!

This is not to say that a GUI interface to the CPAN couldn't be worthwhile. It could be, if done really well. A tree interface for browsing available modules might have potential, for instance, perhaps with POD info about the selected module in another pane. It also might be interesting to be able to select a module and get a tree view of its dependencies, with ones you don't have expanded by default, but the ability to expand even the ones you do have and see the whole tree. And, going a step further, it ought to be possible to select a module in that tree and set options (such as don't run the tests) for installing it.

However, just trying to clone Synaptic but hook it up to the CPAN instead of the deb package system is not, IMO, going to be useful. Much of the way Synaptic is designed is inherently tied to the way the Debian package system is designed, which is rather different from the CPAN. Frankly the Gentoo portage system is more similar to the CPAN than the Debian system is.