Lots of comments, but few solutions. How about this:

After: "Do not have write permissions on '/usr/bin'"

Find the build directory in root's home. It will something like this directory:

/var/root/.cpan/build/CPAN-2.14-XXXXXX

Become root:

sudo -i

and go to the directory:

cd /var/root/.cpan/build/CPAN-2.14-XXXXXX

Edit the Makefile 2 places:

INSTALLBIN = /usr/bin
--->>>
INSTALLBIN = /usr/local/bin
 And:
INSTALLSCRIPT = /usr/bin
--->>>
INSTALLSCRIPT = /usr/local/bin
[download]

Then run make:

make install

This will install cpan (and cpan-mirrors) in /usr/local/bin