You have to print the image to the standard output, but before you have to put this 2 instructions:

1) binmode STDOUT;
2) print "Content-type img/png\n\n"; "Content-type: image/png\n\n"; #as merlyn said

The GD example becomes:

    use GD;
    
    # create a new image
    $im = new GD::Image(100,100);

    # allocate some colors
    $white = $im->colorAllocate(255,255,255);
    $black = $im->colorAllocate(0,0,0);       
    $red = $im->colorAllocate(255,0,0);      
    $blue = $im->colorAllocate(0,0,255);

    # make the background transparent and interlaced
    $im->transparent($white);
    $im->interlaced('true');

    # Put a black frame around the picture
    $im->rectangle(0,0,99,99,$black);

    # Draw a blue oval
    $im->arc(50,50,95,75,0,360,$blue);

    # And fill it with red
    $im->fill(50,50,$red);

    # make sure we are writing to a binary stream
    binmode STDOUT;

    print "Content-type: image/png\n\n"; #as [merlyn] said

    # Convert the image to PNG and print it on standard output
    print $im->png;
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UPDATE: fixed the error in the content type string