Circle not anti-aliased correctly.

Hope this is right list, if not I apologize in advance.

I am using libgd.2.0.0 installed under Ubuntu. Synaptic says libgd2-xpm
V2.0.36 and libgd-dg2-perl V2.39-2

I have generated dg.png using the following perl:
#!/usr/local/bin/perl

use strict;
use GD;

my $gd = GD::Image->new(400,400, 1); # true-color

my $black = $gd->colorAllocate(0,0,0);
my $white = $gd->colorAllocate(255,255,255);
   $gd->setAntiAliased($black);

   $gd->filledRectangle( 0,0, 399,399, $white);
   $gd->line(0,0,399,100,gdAntiAliased);
   $gd->arc(200,200,200,200,0,360,gdAntiAliased);

   open(IMG,">","gd.png");
   binmode IMG;
   print IMG $gd->png;

The result is here:
 http://www.bradcan.homelinux.com/test/gd.png

The line is correctly anti-aliased, but the circle is NOT.

I would have expected the result to look something like this: 
 http://www.bradcan.homelinux.com/test/gimp.png

What's wrong? Did I miss something?

TIA