I found this reading through the Spreadsheet::WriteExcel perldoc, in the Spreadsheet::WriteExcel::Format section.

It looks like you should create a format, then use set_rotation() on it.

my $vertical=$workbook->addformat();
$vertical->set_rotation(1); # letters run from top to bottom
# 2 for 90 degs anticlockwise
# 3 for 90 degs clockwise
[download]

Then just add the $vertical argument to the end of your write calls...

$worksheet->write(0,0, "Title", $vertical);
[download]

Edit: D'oh, missed the merged column part. ++jmcnamara!
--
Mike