use warnings;
use strict;

my $start = '4-Dec-2009';
my $end   = "eof";

print "Date,Total\n";

while (<DATA>) {
    if ( /^$start/ .. /^$end/ ) {
        chomp;
        my (@items) = split /,/;
        printf "%s,%d\n", $items[0], $items[1] + $items[2];
    }
}

__DATA__
Date,Expense,Income
1-Dec-2009,12,87
2-Dec-2009,54,204
3-Dec-2009,75,214
4-Dec-2009,78,198
5-Dec-2009,98,155
6-Dec-2009,10,180
7-Dec-2009,51,91
8-Dec-2009,32,130
9-Dec-2009,29,207