# First we make to test files
% perl -le 'print join(",", map int(rand(1000)), 1..10) for 1..50' > in1.csv
% perl -le 'print join(",", map int(rand(1000)), 1..10) for 1..50' > in2.csv
% head -3 in1.csv
94,434,249,267,649,367,572,579,498,369
452,735,420,543,832,198,28,86,67,382
801,339,978,859,85,719,758,89,191,377

# Now we run dave0's original script on them
perl -lanF, -e '$sum += $F[6]; print "Sum is $sum" if eof' in1.csv in2.csv
Sum is 23789
Sum is 48874

# Now let's try a small change --------------------------.
#                                                        |
#                                                        V
perl -lanF, -e '$sum += $F[6]; print "Sum is $sum" if eof()' in1.csv in2.csv
Sum is 48874