How do I get rid of the pie chart outline in Perl's GD::Graph?

Question

I am trying to create a pie chart with no outline using GD::Graph. Frustratingly I can control the colour of the outline with this:

    accentclr => 'black',

So I would expect that I could get rid of the outline completely by doing this:

    accentclr => undef,

However, when I do this the outline does disappear but the rest of the pie chart does as well with only the labels remaining!

Here's a simplified version of my script:

#!/usr/bin/env perl
use GD::Graph::pie;

# Data to be graphed: 1st array is labels, 2nd array is data
my @data = ( 
     ["1st","2nd","3rd","4th"],
     [ 1, 3.5, 5, 6 ],
  );
my $graph = GD::Graph::pie->new(400, 400);

$graph->set(      
#   accentclr   => undef,   
    '3d'            => 0,
) or die $graph->error;

my $gd = $graph->plot(\@data) or die $graph->error;

open(IMG, '>pie.png') or die $!;
binmode IMG;
print IMG $gd->png;