You appear to have found some buffer limit in either CGI.pm, or more probably, in the AS implementation of that module. The limit appears to come into affect at the boundary between 61439 and 61440 bytes?

This is the limit on my NT system using AS 633. YMMV.

Try the following 4 one-liners

perl -e"print 'x' x 61439;"
perl -e"print 'x' x 61440;"
perl -MCGI -e"print 'x' x 61439;"
perl -MCGI -e"print 'x' x 61440;"
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On my system, the first three fill the screen with 'x's, but the fourth prints nothing at all. I haven't been able to reproduce the exact corruption you experience, but try redirecting the output to a file and you will probably find the corruption gone.

One possible work-around is to output $result in chunks of 60000 chars or so. At least this works on my system

#! perl -sw
use strict;
#$| = 1;
use GD;
use CGI ":all";

my $im = GD::Image->newFromPng("m.png");
my ($width, $height) = $im->getBounds();
my $result;

for my $x (0 .. $width) {
    $result .= "<BR>";
    for my $y (0 .. $height) {
        $result .= font( {
            color=> sprintf "%02x%02x%02x", $im->rgb($im->getPixel($y,
+ $x))
            },'.');
    }
}

print header,$/;
print substr($result, $_*60000, 60000) for 0 .. int(length($result)/60
+000);
print end_html;
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