You are using both the -n switch and <> in the first line! This means you lose half of your lines...

Anyway: -n does an unnecessary chomp on every line, so remove that; and use a limit on split: it doesn't actually need to split all 25 fields:

perl -le 'BEGIN{$,=","} print+(split",",$_,16)[0..14]for <>' numbers.c
+sv
[download]

Update: But for<> reads all lines in at ones; you may not want that with large files, so use while <> instead.