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my @lines = 1 .. 20;
my @interesting = ( 1, 3, 6, 10 , 20 );
...
        if $buf[$viewline] 
           && grep $buf[$viewline] eq $_, @interesting;
}

-->  1 2 3
-->1 2 3 4 5
-->4 5 6 7 8
-->8 9 10 11 12
-->18 19 20