$worksheet->write_rich_string($i, $j, $row_markup);

##</code><code>##

#!/usr/bin/env perl

use strict;
use warnings;

use Excel::Writer::XLSX;

my $workbook = Excel::Writer::XLSX->new( 'test.xlsx' );
my $red = $workbook->add_format( color => 'red' );

print "\$red is a: ", ref($red), "\n";
print "\$red stringifies to: ", $red, "\n";
exit;

##</code><code>##

$red is a: Excel::Writer::XLSX::Format
$red stringifies to: Excel::Writer::XLSX::Format=HASH(0x7f8c741a4f78)