my $buffer;
open my $fh, '>', \$buffer;
#binmode($fh); <- does not seem to make a difference

##############################
# Stop here... 
# I *THOUGHT* it did not make
# a difference, but in fact
# it will, if done correct.
# So the next line of code
# is the solution
#

binmode($fh, ':raw'); # <- Next update: No! It did NOT help.

##############################

my $workbook  = Excel::Writer::XLSX->new($fh);
my $worksheet = $workbook->add_worksheet();
my $sth= database('mydb')->prepare($select);
$sth->execute( @{$query_data->{'params'}} );
excel_results( $worksheet, $sth ); # <- Here I write some data
$workbook->close();
close($fh);

# Now I write out the buffer to the filesystem
open my $x, '>', 'direct.xsl';
binmode $x;
print $x $buffer;
close $x;

# And here I send the file to the browser
return send_file(
    \$buffer,
    content_type => 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
    #content_type => 'application/vnd.ms-excel',
    filename     => 'xxx.xls',
);