in reply to
What is the best way to install CPAN modules on Debian?
If 2) depends on modules authors catering for Debian, I wouldn't use 2). We have all those lovely tools to make CPAN packages, and to download them so module authors do not have to care about the gazillion ways CPAN modules can be installed.
I'd go for 1) if Debian has made that easy. Solaris used to do this right (and probably still do, but I don't have access to a recent Solaris at the moment to verify). Solaris also has tools using Perl. /usr/bin/perl is a link to /usr/whatever/perl. Solaris specific tools start with #!/usr/whatever/perl. This means that if one doesn't install their own perl, one can use #!/usr/bin/perl (because a perl is there). But it also means one can install a /usr/bin/perl without breaking the system tools. If Debian does the same, I'd certainly go for 1). Otherwise, I probably would go for 3).