apache adds extra HTML to an error page (Apache Users)

hello list,

I am trying to migrate a perl script that acts as a webserver from solaris to linux (red hat). The script is run using apache (httpd) and mod_perl.

It nearly does what it is supposed to, exept for the error pages.
When an error occurs and the script can not fulfill the request, it sends a small error page to the user, containing two p elements, together with status 500.

This used to work OK, but on the new machine, apache decides to add extra text to the page. The receiving application sees a page that does not conform to the specification and thinks it is not an error page. This results in various problems.

I have not the slightest idea in what direction I have to search. Is it an apache configuration issue or maybe a perl library issue? I really don't know.
Can you guys give me some clue?

This is the perl code that makes the error page:

 print $cgi->header ( -status => '500 NOT OK',
 -type => 'text/html');
 print $cgi->start_html ('agent');
 print $cgi->p ('ERROR');
 print $cgi->p ('error during database lookup');
 print $cgi->end_html ();

And this is what I receive in my browser:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en-US" xml:lang="en-US">
<head>
<title>agent</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
</head>
<body>ERROR
error during database lookup
</body> </html>
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html>
<head>
<title>200 OK</title>
</head><body>
<h1>OK</h1>
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@xxxxxx and inform them of the time the error occurred, and anything you might have done that may have caused the error. More information about this error may be available in the server error log.
</body>
</html>

thanks in advance, Ruud