There's actually a protocol (Sparkline) for doing this kind of thing - assuming that your graph is relatively simple. In general, though, the previous replies are correct - you can't mix two kinds of MIME data streams under one header.

Example of a sparkline (pie chart) follows:

#!/usr/bin/perl -w
use strict;
use GD::Graph;
use GD::Graph::pie;
use CGI qw/:standard/;
use MIME::Base64;

my @data = (
    ["1st","2nd"],
    [    1,    2],
);

my $graph = new GD::Graph::pie( 120, 120 );

$graph->set( dclrs       => [ split / /, "#FECECE #FEFECE #FFFFFF" ],
             start_angle => 90 );

my $gd = $graph->plot(\@data) or die $graph->error;

print header, start_html,
    h1("Data graph"),
    p('<IMG SRC="data:image/png;base64,',
        encode_base64($gd->png),
        '" alt="sparkline"/>'),
    end_html;
[download]

Also, take a look at GD::Graph::sparklines.

-- 
Human history becomes more and more a race between education and catastrophe. -- HG Wells