From the Spreadsheet::WriteExcel docs (new):

"You can also pass a valid filehandle to the new() constructor. For example in a CGI program you could do something like this:

binmode(STDOUT);
my $workbook  = Spreadsheet::WriteExcel->new(\*STDOUT);
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The requirement for binmode() is explained below. See also, the cgi.pl program in the examples directory of the distro."