I've fallen for the 'cpan install' feature so many times without knowing why that I've just gotten in the habit of using perl -MCPAN. Now that I know why, I'd say the interface of cpan(1) violates the Principle of Lease Surprise, but that's just me. To illustrate:

$ perl -MCPAN -e shell           # Right
cpan> install Foo

$ perl -MCPAN -e 'install Foo'   # Right

$ cpan                           # Right
cpan> install Foo

$ cpan install Foo               # WRONG!
[download]

I think a good compromise would be to special case the distro called 'install' and have cpan(1) refuse to install it unless the -i option is thrown explicitly (or if it is the only item listed, perhaps). And a nice helpful error message if it isn't, such as:

It appears you may be trying to install the 'install' distribution. If this is what you want, use the -i flag as well. If what you really want is to install the '@ARGV[1..-1]' module(s), use the -i flag and omit install. Read perldoc cpan for more information.

And then put a lengthy discussion in the manpage.

I think a good solution would involve the compromise above, plus the deletion of the 'install' distribution, plus the lifetime imprisonment of anyone who ever does or ever has uploaded a distribution called 'install'. But that's just me.