The obvious next step has been re-inventing the wheel. Well, sort-of, because the type of wheel I had in mind wasn't anywhere I was able to look for. In particular, I was looking for some "operating-system command" a-la git that was able to support subcommands for common operations.
So, I concentrated on two sets of commands: the "pure" ones, that focus on Amazon's way of seeing data, and the "filesystem" ones, that support thinking S3 stuff as files with a Unix-like path.
Among the first group you can find the basic S3 functions like add, remove etc. The second group has more "convenient" commands like ls, cp, rm and mv, that try to do the right thing in the spirit of the corresponding Unix commands (e.g. cp can both send or get data, depending on the order of the parameters). I find this useful because the metaphor is somewhat "hardcoded" in me after years of using those commands.
If you're interested or just want to find something to start a flamewar, you can find it at http://repo.or.cz/w/s3.git. Enjoy!
Update: thanks to grink's tip below, I found Term::ShellUI that allowed me to add an interactive shell to call all of the commands, with on-line help! I love CPAN.