To install in NAND memory (if you have the space):
$ sudo opkg install <package-name>
This is just cpan base. You may search for cpan packages there.
If you don't want to fill your NAND (or just can't), you can get the desired packages, unpack them at some folder in your SD card, and setup your perl interpreter to search them at the correct place.
I am just not sure how, cause I'm not into perl, you know