foo.rb

class Foo
  @@count = 1
  def self.count
    @@count
  end
  def self.increment
    @@count += 1
  end
end
Puppet::Functions.create_function(:'test::foo') do
  dispatch :foo do
  end
  def foo
    Foo.increment
    Foo.count
  end
end

output.txt

$ puppet apply ~/puppet_tests/ruby_classvar.pp    
Notice: Scope(Class[main]): 2
Notice: Scope(Class[main]): 3
Notice: Compiled catalog for work-laptop.home.arpa in environment production in 0.01 seconds
Notice: Applied catalog in 0.01 seconds

ruby_classvar.pp

notice(test::foo())
notice(test::foo())