How does $DEBUG variable get set when running a Ruby script

I tried running the script test.rb, containing:

puts "Debugging" if $DEBUG

using the command:

> ruby test开发者_StackOverflow社区.rb --debug

The --debug switch seems to have no effect. What gives?

---

Duh... debug parameter needs to be passed to Ruby.

> ruby --debug test.rb

works as expected.