Are you sure ? I don't see how it can with $rx in single quotes.

You need to provide a Short, Self-Contained, Correct Example like this

#!/usr/bin/perl 
use strict;
use Excel::Writer::XLSX;

my $workbook = Excel::Writer::XLSX->new( 'c:/temp/chart.xlsx' );
my $worksheet = $workbook->add_worksheet();
$worksheet->write_col( 0, 0,[ 'aa'..'zz' ] );
$worksheet->write_col( 0, 1,[ 1..26*26] );

my $chart = $workbook->add_chart( type => 'column' );
my $rx = 101;

$chart->add_series(
   name       => 'HISTOGRAM',
   categories => '=Sheet1!$A$1:$A$'.$rx,  
   values     => '=Sheet1!$B$1:$B$'.$rx,
);
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