I don't think it's invaluable to the modern perl developer. I don't know whether I disagree with your concept of invaluable or your concept of modern perl developer or both. I tried searching "perl repl" on twitter and google to see if there's much talk about it and there doesn't seem to be much. AFAICT it just isn't a big part of the perl workflow, modern or not. Most of the people who like it could manage just fine without it. I don't think the same is the case with python or ruby. Some pythonistas and rubyists use the repl for everything.

I use small files instead of repls most of the time. There's no program that's too short to be stored in a file.