$ ~/.rbenv/versions/2.2.0-preview2/bin/ruby -e '14.instance_eval { p @foo = 14 }'
-e:1:in `block in <main>': can't modify frozen Fixnum (RuntimeError)
  from -e:1:in `instance_eval'
  from -e:1:in `<main>'

$ ~/.rbenv/versions/jruby-1.7.16/bin/ruby -e '14.instance_eval { p @foo = 14 }'
14

$ bin/jruby -e '14.instance_eval { p @foo = 14 }'
14

$ cat ../test.rb 
def Fixnum
  def write
    @foo = 14
  end
end
p 14.write

$ ~/.rbenv/versions/2.2.0-preview2/bin/ruby ../test.rb 
../test.rb:3:in `write': can't modify frozen Fixnum (RuntimeError)
  from ../test.rb:7:in `<main>'

$ ~/.rbenv/versions/jruby-1.7.16/bin/ruby ../test.rb 
14

$ bin/jruby ../test.rb 
14