It sounds to me like the web server saw the image (since it gave you an error message containing part of the image), and the only way it could have seen the image is if it had been sent to STDOUT. Maybe you need to flush the header first?

print "Content-type: image/gif\nExpires: 0\n\n";
{ my $af=$|; $|=1; $|=$af; }  # Flush output.
$rrd->graph( image          => "-",
             vertical_label => 'My Salary',
                       draw => { thickness => 2,
                                 color     => 'FF0000',
                                 legend    => 'Salary over Time',
                        },
    );
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