The you may can use the perl console to install it:
Installation using the Perl Shell
Login to your command line as root and run the following command to start the Perl shell:
perl -MCPAN -e shell
If you run the Perl shell for the first time you will be asked some questions. In most cases the default answers are ok.
Please note: If you run a firewall on your system you might have to turn it off while working on the Perl shell in order for the Perl shell to be able to fetch the needed modules without a big delay. You can switch it on afterwards.
The big advantage of the Perl shell compared to the two other methods described here is that it cares about dependencies when installing new modules. I.e., if it turns out that a prerequisite Perl module is missing when you install another module the Perl shell asks you if it should install the prerequisite module for you. You should answer that question with "Yes".
Run the following commands to install the modules needed by SpamAssassin:
q (to leave the Perl shell)