OS X's default /usr/bin/perl has installsitelib in /Library which is writable, and installsitebin as /usr/local/bin. If a user uses homebrew chances are that they have /usr/local/bin writable as the user.
Yet, some modules (dual-life modules) will install binary and files into /System and /usr/bin and it causes install failures. This happens in OS X Snow Leopard /usr/bin/perl which is 5.10.0. I haven't tested with Lion and 5.12
/Library isn't writable in Lion. Closing.