Unless there's a good reason to generate the HTML and the image at the same time, I would use a seperate CGI to just generate the image data:

# in the HTML generating code:

# set up headers and other html...
print "<img src='/cgi-bin/my-image.cgi?some=params'>";
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and...

# in the /cgi-bin/my-image.cgi script:

# set up the GD::Image in $image
print $cgi->header("image/png"); # set the correct content-type
binmode STDOUT;
print $image->png;
# end
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