1.8.7 output

1.8.7-p330
instance method: Foo::X
singleton method: ::X
class_eval: ::X
instance_eval: ::X

1.9 output

1.9.1-p378
instance method: Foo::X
singleton method: ::X
class_eval: Foo::X
instance_eval: Foo::X
1.9.2-p136
instance method: Foo::X
singleton method: Foo::X
class_eval: ::X
instance_eval: Foo::X

constants.rb

puts "#{RUBY_VERSION}-p#{RUBY_PATCHLEVEL}"

X = '::X'

class Foo
  X = 'Foo::X'

  def foo
    puts "instance method: #{X}"
  end
end

Foo.new.foo

f = Foo.new
def f.bar
  puts "singleton method: #{X}"
end
f.bar

Foo.class_eval do
  puts "class_eval: #{X}"
end

Foo.new.instance_eval do
  puts "instance_eval: #{X}"
end