If you want to get rid of the 'cut', you can do it all in Perl using the -a and -F switches (see perlrun for details) to autosplit the input line into the @F array:
perl -lanF, -e '$sum += $F[6]; print "Sum is $sum" if eof' in.csv
Thanks dave! I have never used aF switches before. I shall remember to use this instead of my split/,/; However I am not sure if perl's split is more efficient than cut... I guess I can do a quick field test on a large file!