While debugging, if you ever need to write to $stderr you might use $stderr#puts, but you can use Warning#warn which is better called from Kernel, because Kernel appends newlines and respects the -W0 flag:
$stderr.puts "you have been warned"
Warning.warn "you have been warned"
Kernel.warn "you have been warned"
warn "you have been warned"
Output:
you have been warned
you have been warnedyou have been warned
you have been warned