IRB gives you an outer loop, which means that the ruby process doesn't exit until you decide to kill it. The problem with your ruby script is that the main process is finishing and killing your child (yikes) before it has the chance to trap the signal.
My guess is that this is a test script, and the chances are that your desired program won't have the case where the parent finishes before the child. To see your trap working in a plain ruby script, add a sleep at the end:
pid = fork {sleep 2; puts 'hello'}
trap('CLD') { puts "pid: #{pid} exited with code"}
sleep 3
To populate the $?
global variable, you should explicitly wait for the child process to exit:
pid = fork {sleep 2; puts 'hello'}
trap('CLD') { puts "pid: #{pid} exited with code #{$? >> 8}" }
Process.wait
If you do want the child to run after the parent process has died, you want a daemon (double fork).