$x = "a"
$, = "b"

alias $old_slash $,
alias $, $x

p $,

puts "---"
print '123','456'               # 123b456 -> ignores alias
puts "","---"

p $,.object_id == $,.object_id

alias $, $old_slash       # restore $,

puts '-' * 20

$, = nil                           
($, = 1) rescue puts $!

class S
  def to_s; "bob"; end
  def to_str; "bob"; end
end

($, = S.new) rescue puts $!