... and I'm finally getting around to providing an example today:

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>
[download]

use XML::Rules;

my $parser = XML::Rules->new(
  style => 'filter',
  rules => {
    article => sub {
      my ($tag,$attr) = @_;
      $attr->{_content} =~ s/EM/E/g;
      return $tag => $attr;
    }
  }
);

$parser->filterfile( 'a.xml', 'b.xml');
[download]

Thanks for all the replies. I am trying to replace the text ctrl+O which is ^O in the file.

open FH,'a.txt';
@data=<FH>;
foreach $dat (@data){
$dat =~ s/\^O/ /g;
}
[download]

I am trying to replace the control characters with a space but no luck. Please can you suggest me how to proceed

Perl doesn't understand what do you mean by that \^O. Try \x0F instead.

Jenda
Enoch was right!
Enjoy the last years of Rome.

s/EM(?=[^<]*</article>)/replacement/g.

This will match and replace any occurence of the letters EM that are followed by any number of non-< characters and then a closing article tag.