Just remove the call to decode:

      $worksheet->write($rowCount, $colCount, $Field, $gFmtBold);
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I also had to remove the following lines from the script to test it (they're irrelevant to the question, anyway):

use Locale::Msgcat;
use Number::Localization;
use Presentation;
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And I had to prepend

use Excel::Writer::XLSX;
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See Short, Self Contained, Correct Example on how to help us to help you.

See the module's documentation on the details:

If the data is in UTF-8 format then Excel::Writer::XLSX will handle it automatically.