my @source = (1, 2, 3,  4);

##</code><code>##

       my @pair  = (1,2);   
       my @pair2 = (2,3);  
       my @pair3 = (1,3);
       my @pair4 = (2,4);

##</code><code>##

    @pair -> []
    @pair2 -> [1]
    @pair3 -> [2]
    @pair4 -> [1,3]

##</code><code>##

    sub get_output {
         my ($source,$pair) = @_;
    
      my @output = ();
      my %done = ();
      foreach my $pr (@{$pair}){
         foreach my $sc (@{$source}){
                 next if ($pr <= $kn || $done{$sc});
                 push @output,$sc;  
                 $done{$sc} = 1;
          
         }
      }
    
      use Data::Dumper;
      print Dumper \@output;
      return @output;
    }