foo.rb

require 'open3'

i, o = Open3.popen2 'ruby repl.rb'
line = nil

puts 'Gimme Ruby expressions to evaluate (in another process).'

Thread.new do
  i.puts line while line = gets
  i.close
end

puts line while line = o.gets

repl.rb

while line = gets
  p eval line
  $stdout.flush
end