You didn't mention that you were using Embperl 2 before. Embperl can flush buffers, but Embperl 2 is a totally different system.

Sorry about that.

What's confusing about your diagram is what you mean by "redirect" in #2.

I mean simple redirect on Embperl script which call object method implemented long running operation. The browser will wait untill operation has finished because Embperl doesn't flush buffers till finish of page generation.

The forking approach that merlyn described is a bit more complex, but better for scalability.

I don't have pretensions to say that my approach is much better than merlyn's. ;))
I'd like to show your other way to do that. So in my case, this approach is better for me. I suspect that the best way to do wait message is implementing forking approach in the mod_perl handler. But I don't have enough knowledge of mod_perl and time to do that.