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

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

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

    sub get_output {
         my ($source,$pair) = @_;
...
    }