What is pvt.cppan.demo namespace?

pvt.cppan is a utility CPPAN account and pvt.cppan.demo contains many projects, but they're not official. You could use them as a starting point in uploading your packages. And official packages will be in pvt.your_name.* namespace. Also for orgs: org.boost.*, org.google.*, org.facebook.*, com.ibm.*.

Where does cppan download/generate stuff?

By default, in your user home directory. Like $HOME/.cppan/storage on *nix or %USERPROFILE%/.cppan/storage on Windows.

How to change that directory?

Open $HOME/.cppan/cppan.yml, add storage_dir: your/favourite/dir/for/cppan.