Thank You!!!

So I did not use your suggestion of using quotemeta (I haven't used it before, so I'm gonna go look it up), but I got my answer in the sample output you linked :)

Here's my updated code that's working now. Apparently I just needed to preemptively escape the . in my search string.

my $file = "foo.c";

$file =~ s/\./\\\./;

`perl -p -l -i.bak -e "s/$file/$file,=SUM(B$x:B$y)/" sample.csv`

Though probably I should look into Dean's suggestion as well. Any thoughts on which approach would have the fastest execution time out of these?