As GotToBTru said, your code isn't very useful without sample data.

But if I have understood your problem well, there is one obvious solution. Uniqueness means hash:

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

existsPair "Hello", "World"; # false
existsPair "Hello", "World"; # true
existsPair "World", "Hello"; # true
existsPair "He", "lloWorld"; # false
[download]

The hardest part of the job is to find a way to stringify the pair in a way that is actually unique, it can be by using an invalid character as a separator, or using pack for prepending the size of the first string, or whatever works.