So it turns out that currently ruby-debug [the *great* debugger for 1.8.x] isn’t quite compatible with 1.9
Are we lost?
Never!
Some things you could try
1) run it with ruby’s normal debugger -rdebug [1] kind of slow
2) run it with unroller gem [kind of stinks, way slow]
3) use a lot of print statements 
fast, kind of less introspection possible
4) drop to an irb prompt and run some code from there.
you could list the code by creating your own “drop to irb prompt” that listed the code around itself [use caller to discover where you where] then drop to a normal irb prompt.
5) maybe jruby in 1.9 compat mode has a debugger?