If they are already sequential, subtract 8.

print $columns[$n]-8 ,"\n";
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Or adding a counter is reasonable. Since you don't change $n, you could repurpose that as your counter and change the data you started with.

#for testing:
$columns[1] = $n++;
print "$columns[1] \n";

# Print the whole array with new numbering
print "@columns\n"
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or without changing the original data, use the postincrement operator

#for testing
print $n++,"\n";
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