I was going to post the same thing - until I tested it. When you pass 'false' to the excel object, it treats it as a False value.

Quick test:

use strict;
use warnings;
use Win32::OLE;

my $excel = Win32::OLE->CreateObject('Excel.Application');
$excel->{Visible} = 1;
sleep 5;
$excel->{Visible} = 'False';
sleep 5;
$excel->{Visible} = 1;
print "press enter...";
<STDIN>;
[download]

Excel will appear, then disappear for 5 seconds, then reappear.