Why piping to ruby? You have the -e operator for that:
ruby -e 'loop do print ["\u2571","\u2572"].sample ; sleep 0.001 end' ruby -e 'loop do print ["\u2571","\u2572"].sample ; sleep 0.001 end' ruby -e 'print %W{\u2571 \u2572}.sample while sleep 1e-3'
(Updated: Made 3 chars shorter.)And a shorter, but cheats, version:
yes|ruby -pe'$_=%W{\u2571 \u2572}.sample;sleep 1e-3'
My favorite overall balance of readability and shortness though is: ruby -e 'print %w{╱ ╲}.sample while sleep 0.01'