Do the child threads block the main thread from executing until they are "join"ed?

No.

B) If no, then, why am I not seeing any output/activity from the main thread?

Without seeing your code it is difficult to say. A common issue is buffering. That is, you cannot see anything happening because output is being written to an in-memory buffer which does not get flushed until it is closed, unless the program explicitly flushes it. If you make STDOUT unbuffered using local $|=1; then that might help.