It looks like you are using the wrong value for the row. Try this example:

#!/usr/bin/perl
use strict;
use warnings;
use Spreadsheet::WriteExcel;
my $workbook = Spreadsheet::WriteExcel->new("test.xls");
my $worksheet = $workbook->addworksheet();
$worksheet->write('C20', 10);
$worksheet->write('D20', 20);
$worksheet->write('E20', 30);
my $row = 20;
# Both of the following methods work
# this first one you have to subtract 1 from $row
#$worksheet->write($row-1, 6, "=SUM(C$row:E$row)");
# this one you don't
$worksheet->write("G$row", "=SUM(C$row:E$row)");
$workbook->close();
[download]