The value of VBA's xlCenter is not the string 'xlCenter'. You will want to see Win32::OLE::Const for how to extract the xl-constants.

Based on your last example, I think you would need to add

use Win32::OLE::Const;
my $xlConstants = Win32::OLE::Const->Load($Excel);
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and then change to

$Sheet->Range("B1:Q1")->{HorizontalAlignment} = $xlConstants->{xlCente
+r};
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(untested: sorry, I don't have Excel on this machine)