sub existsPair
{
  our %pairs;
  my (@pair) = sort @_;
  local $" = "~~"; # Only works as long as ~~ can't appear in one of the inputs values
  return $pairs{"@pair"}++;
}

existsPair "Hello", "World"; # false
existsPair "Hello", "World"; # true
existsPair "World", "Hello"; # true
existsPair "He", "lloWorld"; # false