You seem to be treating $red like it is a scalar value that contains a string. This is not the case. Your $red variable contains a Excel::Writer::XLSX::Format object. The write_rich_string function takes a mixed list of strings and Excel::Writer::XLSX::Format references as arguments and then constructs the formatted string that is placed in the worksheet.

In your code, this line:

$worksheet->write_rich_string($i, $j, $row_markup);
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will result in the string contained in $row_markup being placed in the worksheet verbatim.

Here is a bit of code that might help clear things up:

#!/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;
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Output:

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