Special variable $1 getting clobbered from another thread

threadweirdness.rb

class ProcCaller
  def add(&block)
    (@procs ||= []) << block
  end

  def callem!
    @procs.map do |p|
      Thread.new { p.call }
    end.each &:join
  end
end

pc = ProcCaller.new

pc.add do
  "foobar" =~ /(.)ar/
  puts "$1 is #{$1.inspect}"
  sleep 0.4
  puts "$1 is now #{$1.inspect}"
end

pc.add do
  sleep 0.2
  "x" =~ /(.)/
  puts "boop!"
end

pc.callem!

# $1 is "b"
# boop!
# $1 is now "x"