use strict;
use warnings;
use XML::Twig;

my $xmlStr = <<XML;
<foo>
    <article>ME ME ME</article>
    <particle>ME TOO</particle>
</foo>
XML

my $twig= new XML::Twig(
    twig_handlers => { article => \&article }
);
$twig->parse($xmlStr);
$twig->print();
print "\n";
exit;

sub article {
    my ($twig, $art) = @_;
    my $stuff = $art->text();  # get the text of article
    $stuff =~ s/ME/E/g;
    $art->set_text($stuff);
}  

__END__
<foo><article>E E E</article><particle>ME TOO</particle></foo>