Now it is my machine that is the slowpoke; still, for me, the internal pipe continues to fare best:

% time perl -lne 'BEGIN{ $,=","}; print+(split ",")[0..14]' numbers.cs
+v \
> /dev/null
12.65s user 0.01s system 98% cpu 12.890 total
% time perl -le 'open IN, q(cut -d, -f"1-15" numbers.csv|); \
print join ",", ( chomp and split /,/ ) while <IN>' > dev/null
8.17s user 0.01s system 90% cpu 9.070 total
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