Thank you for your help. I have found another problem and then subsequently a solution for the writing vertically in a merged column. The trick is to write what you want to the destination cell and then merge the cells. If you merge them first and then write the data to the beginning cell of the merged column it only registers as being written to that cell and it is treated as though it is in a single cell rather than a merged column. Here is the working code:

#!/usr/bin/perl -w

use strict;
use Spreadsheet::WriteExcel;

# Variables
   my $file_name = 'switch_report.xls';
   my $sheet_title='Traffic Estimations Report -May 2002';
   my @array=(    ['Switch 1','','above cell blank','456','789'],
        ['Switch 2','DEF','GHI']
           );
   #print $1;
   #my $array_ref=\$1;
   my $array_ref=\@array;



# Create a new Excel workbook
   my $workbook = Spreadsheet::WriteExcel->new($file_name);

# Add a worksheet
   my $worksheet = $workbook->addworksheet();
   $worksheet->fit_to_pages(1,1);
   $worksheet->set_landscape();
   $worksheet->set_margins(.25);
# Add formats
   my $cb_format = $workbook->addformat(); # Add a format
   my $rotation_format = $workbook->addformat();

# Define Formats
#  $cb_format
   $cb_format->set_bold();
   $cb_format->set_italic();
   $cb_format->set_underline();
   $cb_format->set_align('center');
   $cb_format->set_size(18);

# $rotation_format
   $rotation_format->set_rotation(2);
   $rotation_format->set_bold();
   $rotation_format->set_align('vcenter');


# Write to the worksheet
   $worksheet->write(0,1,$sheet_title, $cb_format);
   $worksheet->merge_cells(0,1,2,29);
   $worksheet->merge_cells(3,1,3,29);

#$worksheet->write_row(11,1,$array_ref);

   $worksheet->write(4,0,"Day of the month",$rotation_format);
   $worksheet->merge_cells(4,0,37,0);


# write_switch_data
   my $row = 4;
   my $col = 1;
   $worksheet->write_row($row, $col, $array_ref);
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