I think this module is very problematic. It is not really documented clearly, and it tries to do too many things, and is generally far too clever for its own good. Just now, as a test to see if a problem had occurred, I tried to use this to install a module which had not been installed via cpanm. The cpan shell started lots of background processes, tried to locate some kind of mirror in Singapore at "choon.net", kept on trying to download again and again after repeated failures, messed up my terminal, could not be stopped from downloading via interrupts, but kept on and on spawning background processes after one presses the interrupt. Despite all the help and the options on the screen if I press h, it won't tell me the simple way to do things like how to stop it connecting to the unavailable mirror at choon.net and connect to another CPAN mirror. Looking at google for help there are threads from 1994 and 2004 full of "weird old tricks" on how to configure this thing. 
The only thing that 99.9% of the people using this 99.99% of the time ever needed was a way to download modules and install them. What seems to have happened is that the author started adding all the bells and whistles for fun or something, and ended up making it unable to do the simple job that people actually need it to do.
The documentation is appalling. The functions are not actually explained - you have to guess what they do or read the source code. An explanation of what "readable" is trying to do would be especially helpful. Also 99 open bugs is sort of scary. Apart from that it does what it says on the tin - it's just that the list of ingredients, countries of origin and healthy warnings, salt levels etc etc are missing.
I've been using CPAN for years and find it invaluable in interfacing to the CPAN archive. The few times I have had problems, it has always been with either missing system programs (like wget) or networking problems, not the module. The documentation is detailed and well worth taking the time to read and learn all the options. Thank you Andreas!
CPAN.pm was of so high quality for so many years that I simply want to say Thank You for this module.
Some time ago I very often heard that CPAN.pm had too difficult internals, everything would be quirky and unmaintainable, etc. This might be true, I don't know and I don't care, because CPAN.pm (or better say its author) *never* let me alone. It always worked very robust for me on different platforms and with practically every version of it I tried.
It especially worked much more reliable than its designated successsor CPANPLUS. I tried to switch to CPANPLUS some years ago but switched back to CPAN.pm simply because of the much higher reliability.
And since the once stalled development resumed with support for Module::Build, CPAN::Reporter, etc., my CPAN world is in perfect balance.
Used from the command line or via its own shell for searching for and installing perl modules. Requires almost no technical knowledge, and as such, a great way for end-users to bring limited parts of a distribution up-to-date.