According to the docs, the value needs to be one of the XlHAlign values. Note that XlHAlign is an enumeration, so the values are numbers. "Left" is definitely wrong. Constants can usually be imported using Win32::OLE::Const.

use Win32::OLE::Const 'Microsoft Excel';

...

$sheet->Range($range)->{HorizontalAlignment} = xlHAlignCenter;
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Untested.