# Number of elements in the array are always EVEN
my @arr = qw (J K L M N O);
my %done;

OUT:
foreach my $elem(@arr) {

   IN:
   foreach my $elem2(@arr) {
   
      next IN if ($elem eq $elem2);
      next IN if ($done{$elem2." - ". $elem});

      print "$elem - $elem2\n";
      
      $done{$elem." - ".$elem2} = 1;

    }
 
}