Turns out I needed to control the "alpha" channel with the fill. The code looks like this:
<snippet>
$image->Draw( fill=>'rgba(0,0,0,0.55)', primitive=>'rectangle', points
+=>'1460,1000 2060,1180');
$image->Annotate( gravity=>'southeast', font=>'Arial', pointsize=>18,
+fill=>'red', x=>500, y=>100, text=>"Quake Information Last Updated
+at: Mon Jul 1 21:09:46 2013" );
$image->Annotate( gravity=>'southeast', font=>'Arial', pointsize=>18,
+fill=>'green', x=>500, y=>125, text=>"NORAD Information Last Update
+d at: Mon Jul 1 21:09:46 2013" );
</snippet>
