#declare a global:
my $only_one=0;

#same as before, then:
twig_handlers => {   tm=>sub{my $text = $_->text();   $only_one ? $_->cut()  : ( $_->cut() and print $text and $only_one++ ) },

__DATA__
<?xml version="1.0"?>
                <stats>
                  <tm tmclass="ibm" tmowner="IBM Corporation" tmtype="reg" trademark="AIX">AIX</tm>
                  <tm tmclass="ibm" tmowner="IBM Corporation" tmtype="reg" trademark="AIX">AIX</tm>
                  <tm tmclass="ibm" tmowner="IBM Corporation" tmtype="reg" trademark="AIX">AIX</tm>
                </stats>'

__OUTPUT__
<?xml version="1.0"?>
                <stats>
                  AIX


                </stats>