add_format(%properties)
  The "add_format()" method can be used to create new Format objects which
  are used to apply formatting to a cell. You can either define the
  properties at creation time via a hash of property values or later via
  method calls.

      $format1 = $workbook->add_format(%props); # Set properties at creation
      $format2 = $workbook->add_format();       # Set properties later

  See the "CELL FORMATTING" section for more details about Format
  properties and how to set them.