If you are not changing the name then use Save not SaveAs(newname)

#!perl
use strict;
use warnings;
use Win32::OLE;
$Win32::OLE::Warn = 3;  

my $filename = 'c:/temp/Book2.xlsx';
 
my $excel = Win32::OLE->new('Excel.Application', 'Quit');
my $book = $excel->Workbooks->Open($filename)
  or die Win32::OLE->LastError();
my $sheet = $book->Worksheets('sheet1');
$sheet->Range("A1")->{Value} = localtime; # update something
$book->Save;
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