I too suffered from this malady. Here's what I did.: (This is available on macosx_perl mailing list as well)

Here's what I did.  I followed the advice of Elaine -HFB- Ashton and l
+ooked at my CPAN Config.pm file in it I found that _MY_ Config.pm fil
+e had "blanks" next to important settings like tar, unzip and make.  
+Filling in those blanks fixed CPAN.

Here are the steps I took:

cd /System/Library/Perl/CPAN
sudo perl -MCPAN -e shell

At the CPAN shell I typed
o conf

o conf allowed me to look a my configuration.  If you see things like:

make

then you need to adjust those settings.  CPAN can't find make.  That's
+ bad.  To figure out where these programs are you can type:

which make

Which should return:

/usr/bin/make

Mostly the programs listed in o conf should be in /usr/bin but I would
+ check.

To edit Config.pm you'll need to do something like this:

sudo emacs Config.pm  (choose the editor of _your_ choice)

You'll now see entries like:

'make' => q[ ],

You'll want them to look like this:

'make' => q [/usr/bin/make],

Not every entry in my Config.pm has a setting.  For instance I don't h
+ave lynx, and so there's no entry.  Keep two terminals open while you
+ do this.  So if here is an entry and you're not sure about it you ca
+n do a which #### on the entry to see where/if you have it.

Good luck

Oh and if  you choose emacs CTRL-S will save and CTRL-C will get you o
+ut of emacs.
[download]

The OP of that thread was a newbie who had a lot of trouble with even the text editor so please forgive all the emacs advice salt and peppered thoughout.
Sorry about the formatting but I was too lazy to make it pretty and I love using the code tags!
--
ellem@optonline.net
There's more than one way to do it, just don't use my way.