#$num = '##,##0'; # this does not work

The following code works with '#,##0'. This was copied from the cpan page and modified slightly.

use strict;
use warnings;
use Excel::Writer::XLSX;

# Create a new Excel workbook
my $workbook = Excel::Writer::XLSX->new( 'perl.xlsx' );

# Add a worksheet
my $worksheet = $workbook->add_worksheet();
my $format02 = $workbook->add_format();
my $format03 = $workbook->add_format();
    
$format02->set_num_format( '#,##0' );
$worksheet->write( 1, 0, 1234.56, $format02 );      # 1,235
$worksheet->write( 2, 0, 123456789, $format02 );      # 123,456,789

$format03->set_num_format( '#,##0.00' );
$worksheet->write( 3, 1, 1234.56, $format03 );      # 1,234.56
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Update: I used set_num_format per Corion's suggestion.

Update: poj has modified Re: Need help in Excel::Writer::XLSX to add the link to the Wikipedia page Indian numbering system after I added this link in my other reply.