It looks to me like any mention of 'now' in the parent (maybe excluding children of the parent) would make the entry not rare. So I would check just that: whether you can match 'now' in the parent's text.

You can test your code by building a quick test environment like the one below, that will let you add headers and check that your sub behaves properly:

 #!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;
use Test::More;

XML::Twig->new( twig_handlers => { header => \&test_header })
         ->parse( \*DATA);

done_testing();

sub test_header
  { my( $t, $header)= @_;
    is( $header->att( 'expected'), is_la_rare( $header), $header->spri
+nt);
  }

sub is_la_rare {
    my $elt = shift;
    foreach my $label ($elt->children('la') ) {
        next unless $label->text eq "rare";
        my $isNowRare = 0;
        if ( $label->parent->text_only =~ m/[nN]ow / ) { $isNowRare = 
+1; }
        if ( !$isNowRare ) { return 1; }
    }
    return 0;
}

__DATA__
<tests>
  <header expected="1"><la>rare</la></header>
  <header expected="0">Now <la>rare</la></header>
  <header expected="0">Now <la>hist.</la> and <la>rare</la></header>
</tests>
[download]

And it looks like xmltwig.org was down, it's back to life now, !@#$%^&* networks! Sorry for the inconvenience