Because it can do things that CPAN does not.
For example, I can install a distribution on disk extremely easily using CPANPLUS. "cpanp install file:///c:\MyDist-1.01_01.tar.gz" does it for me in one step. Useful when installing a dist on a virtual machine.
I don't see how to do that using the CPAN.
Yes, it can be improved, and should be. But it's very good as is!
It has a zillion dependencies, and nets you a command shell just about as cryptic and powerless as regular CPAN. Why bother?
CPANPLUS::Backend / CPANPLUS (0.051)
CPANPLUS::Backend does not make it easy to automate CPAN-related tasks (such as running smoke tests on new CPAN modules).
It's not well documented, and the interface is too limited. To get a minimum script running requires quite a bit of hacking and using undocumented or private (names beginning with underscores, "_") methods.
For instance, it's hard to dig into a distributions prerequisites in order to determine if there's something that you know doesn't work and have it bail out. Or maybe have it fall back onto a previous version if the latest version fails a test. (It shouldn't be so hard to delve into the inner-workings of the module with some well-placed callbacks. But maybe this is possible, but just not documented?)
In certain cases it cannot properly parse the path to the module, requests the wrong URL and just locks up unsure of what to do. (Possibly a Windows-issue with time outs?).
The tracking of sent test reports seems to have been eliminated, which improves usage from the CPAN Shell but puts the onus on smoke testers to track results.
My frustration has tempted me to just roll my own fetch/extract/run Makefile.PL/make test and send reports code myself, but I do realize what a complicated mess it really is to handle the disgusting underbelly of module installation (especially in a platform-independent manner).
So in a perverse way I am grateful for all the work on it. This is an enormous effort so I have high hopes that some day it will be improved.
CPANPLUS gets my vote for "best new module of the year". CPANPLUS is a replacement for the venerable CPAN.pm, and is scheduled for addition in the 5.10 core. CPANPLUS makes installing CPAN modules trivial, and it even supports uninstallation. Developers will appreciate CPANPLUS::Backend, which allows a programatic interface to its functionality. If you are at all serious about perl, this is one must-have module.
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