pacman.rb

class Pacman
  attr_accessor :screen
  def initialize(starting_screen)
    @screen = starting_screen
  end

  def evolve!
    if (dash_index = @screen.index '-')
      @screen[dash_index] = '<'
    elsif (dot_index = screen.index '.')
      @screen[dot_index] = '-'
      @screen[dot_index-1] = ' '
    else
      @screen = nil
    end
  end
end

pacman = Pacman.new('<.....................')
while pacman.screen
  print pacman.screen + "\r"
  pacman.evolve!
  sleep 0.3
end

pacman_test.rb

$:.unshift File.dirname(__FILE__) + '/../lib'

require 'minitest/autorun'
require 'pacman'

class PacmanTest < Minitest::Test
  def evolve screen
    pacman = Pacman.new(screen)
    pacman.evolve!
    pacman.screen
  end
  def test_end
    assert_equal nil, evolve('XXX>')
  end

  def test_open_mouth
    assert_equal '<', evolve('-')
    assert_equal ' <', evolve(' -')
  end

  def test_eat_one_dot
    assert_equal ' -', evolve('<.')
    #             1234           1234
    assert_equal '  -.', evolve(' <..')
  end
end

Run with
ruby pacman.rb

You'll have to imagine the "waka-waka" sound :)