Hi,
by chance I have the solution for this in my code sample at Avoiding Strange Win32::ODBC Return Values for Integers (NUL).
Look at the parts

#  Add and define a format
my $header = $workbook->addformat(); # Add a format
...
$header->set_bg_color('yellow');
...
$worksheet->write($row, $col++, $_, $header) for @fields;
[download]

Cheers,
jonix