foo.rb

class Foo
  foo = %w(a b c)
  BAR = %w(a b c)

  foo.each do |method|
    define_method(method) { p method.upcase! }
  end

  BAR.each do |method|
    define_method(method.upcase!) { p method }
  end
  p foo
  p BAR
end

Foo.new