How can I start an interactive console for Perl?

You can use the perl debugger on a trivial program, like so:

perl -de1

Alternatively there's Alexis Sukrieh's Perl Console application, but I haven't used it.

Not only did Matt Trout write an article about a REPL, he actually wrote one - Devel::REPL

I've used it a bit and it works fairly well, and it's under active development.

BTW, I have no idea why someone modded down the person who mentioned using "perl -e" from the console. This isn't really a REPL, true, but it's fantastically useful, and I use it all the time.