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s/(?<=\d)(?=(?:\d{3})+\b)/$sep/g;
substr($n, pos($n), 0) = $sep while ($n =~ /\d(?=(?:\d{3})+\b)/g);
substr($n, -($_*3 + ($_-1)*length($sep)), 0) = $sep for (1..int((lengt +h($n)-1)/3));