[ruby-talk:16782] strange behaviour with fork and set_trace_func

After the discussion yesterday about getting the caller's binding, I've
been playing with the idea of writing a caller_binding() function with
pure Ruby.  I'm pretty close, but I've hit an interesting roadblock:

def bar(x)
  if not fork then
    set_trace_func(proc{|*a|
      if a[0] == "return" then
        # begin
          expr = x.to_s
          puts eval(expr, a[4])
        # rescue Exception
        #  puts $!
        # end
        exit!
      end
    })
  else
    Process.wait
  end
end

def foo()
  bar("a")
end

a = 42
foo()

The code above prints "42".  However, if I remove the comments and catch
the exception, then I get:

(eval):1:in `bar': undefined local variable or method `a' for
#<Object:0x40282c90>

Why am I not getting the exception when it is not explicitly caught?

Paul