Ruby 1.9.3 seems to supports the syntax:
a = b = f 1
In JRuby it should be written as:
a = b = f(1)
It's weird, but it's true. This is by design. https://github.com/ruby/ruby/blob/trunk/parse.y#L1228-1246
This guy needs a RubySpec.
In the meantime, I have fixed this on the master branch: 1dd08ac