A followup: my ranges are quite large (~500k numbers, usually in one or two blocks only, i.e. something like 1..20000,700000..1000000).

What is the efficient way to simply print them out to a file?

Maybe  for $i (1..20000,700000..1000000) {print "$i "}? I'm not sure if an array is actually created here... if it is, maybe it's not a good idea.