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',
                        },
    );

[download]

In reply to Re: Tricking a filename argument to use STDOUT by ikegami
in thread Tricking a filename argument to use STDOUT by fapestniegd

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.