I believe you can set the brush properties and draw with said brush to create meaty arcs (untested)

#!/usr/local/bin/perl

use GD;

# Create a flat wide rectangle paintbrush
$brush = new GD::Image(10,10);
$brush->colorAllocate(255,255,255); # white
$brush->colorAllocate(0,0,0);   # black
$brush->transparent($white);    # white is transparent
$brush->filledRectangle(0,0,5,2,$black); # a black rectangle

# Draw an oval
$im->setBrush($brush);
$im->arc(100,100,100,150,0,360,gdBrushed);
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-Mark