According to the author, the use of @data is legacy code. I rewrote it to reflect the newer usage. Note that the usual way to print is...

open(IMG, '>test.png');
binmode IMG;
print IMG $gd->png;
[download]

The full script:

#!/usr/bin/perl -l

use strict;
use warnings;
use GD::Graph::bars;
use GD::Graph::Data;

print "Hello world!\n";

my $data = GD::Graph::Data->new([
    [1,2,3,4], 
    [5,3,8,9],
]) or die $!;

my $graph = GD::Graph::bars->new();
$graph->set(
    x_label => 'X num',
    y_label => 'Y num',
    title   => 'Test',
) or die $graph->error;

my $gd = $graph->plot($data) or die $!;

open(IMG, '>test.png');
binmode IMG;
print IMG $gd->png;
[download]