The module Statistics::Descriptive is a Perl CPAN module, and how you add a Perl module will change depending on the flavor of Perl you are running.

Are you running Perl on a Windows box using Active State Perl or on a Unix machine?

In Active State, you would use the command 'ppm' to launch the tool to install modules, and in Unix, you can use the command:

perl -MCPAN -e shell
[download]

To start the process of accessing the CPAN repository. If you do not have Internet access from your machine, you may have to download the modules manually.

If you need to know more, there is this node on Perl Monks that has all kinds of information about installing modules.

Update: added PM node on module installation.