I ran into a bit of a problem with this recently too. This is what I had. It worked, but, with warnings on, it threw a warning about using an undefined value in a le comparison.

use CGI;
my $cgi = new CGI;
#...more stuff not important to this discussion
print $cgi->header( -type => 'application/xml');
print '<?xml version.......blah blah'."\n";
[download]

My browser seemed to know what to do with it just fine.

In order to get around the warning, I stopped using the cgi header method. This is now what I do.

use CGI;
my $cgi = new CGI;
#...more stuff not important to this discussion
print "Content-Type: application/xml\n".
      "\n".
      '<?xml version.......blah blah'."\n";
[download]

I found that I had to have exactly two \n characters after the Content-Type, before the "<?xml" part. Any more or less and the browser complained about not being able to parse the xml even though applications looking at the xml could handle it.