Some of the slowness appears to be related to the split, rather than the join. On my system, this takes about 9s: 
$ time perl -lanF, -e 'print join ",", @F[0..14];' numbers.csv > /dev/
+null

real    0m9.880s
user    0m9.716s
sys     0m0.034s

[download]
and your version takes a little less: 
$ time perl -lanF, -e 'BEGIN{ $,=","} print @F[0..14];' numbers.csv > 
+/dev/null

real    0m8.974s
user    0m8.772s
sys     0m0.042s

[download]
but this one avoiding both -a and join only takes about 3.4s: 
$ time perl -ln -e 'print $1 if /((?:[^,]+,){14}[^,]+)/' numbers.csv >
+ /dev/null

real    0m3.412s
user    0m3.370s
sys     0m0.031s

[download]

In reply to Re^2: cut vs split (suggestions) by dave0
in thread cut vs split (suggestions) by sk

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