Your code doesn't look much like the sample in John McNamarra's doc... where it says:

add_format(%properties)
  The "add_format()" method can be used to create new Format objects w
+hich
  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 v
+ia
  method calls.

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

  See the "CELL FORMATTING" section for more details about Format
  properties and how to set them.
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And before you drill all the way down to "CELL FORMATTING" read this:

  One problem with the "write()" method is that occasionally data look
+s
  like a number but you don't want it treated as a number. For example
+,
  zip codes or ID numbers often start with a leading zero. If you writ
+e
  this data as a number then the leading zero(s) will be stripped. You
+ can
  change this default behaviour by using the "keep_leading_zeros()"
  method. While this property is in place any integers with leading ze
+ros
  will be treated as strings and the zeros will be preserved. See the
  "keep_leading_zeros()" section for a full discussion of this issue.
 More  -- ...
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So, while I understand that your problem is not leading zeros but rather, the conversion of text to a number in scientific notation, the docs may help there...

But here's where your my $textFormat = $workbook->add_format( num_format => 'Text' ); seems awry: you don't want to set a number format; you want simple text, if I've understood your dilemma correctly.

 See the note about "Cell notation". The $format parameter is optional

 In general it is sufficient to use the "write()" method.

rite_string($row, $column, $string, $format)
 Write a string to the cell specified by $row and $column:

     $worksheet->write_string(0, 0, 'Your text here' );
     $worksheet->write_string('A2', 'or here' );
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So unless the conversion is occuring as you READ the source data, it would appear that further reading in the doc (ie, perldoc Spreadsheet::WriteExcel will help you solve this issue.

Alternately, you might want to prepend a single-quote to the $var ($var presumed here) you're trying to write to the new sheet.