#!/usr/bin/perl -wT
use strict;
use CGI;
my $q = new CGI;
if ( grep { /application\/xhtml\+xml/ } $q->Accept ) { print "Content-type: application/xhtml+xml\n\n"; }
else { print "Content-type: text/html;Charset=utf-8\n\n"; }
print 'some xhtml';
...