TX for your reply. I have implemented some codes but I have problem for replace the REF element with EXT element. This is the original file

<DOC id="AFP_ENG_20050316.0102" type="story">

<HEADLINE>

Bobby Fischer can escape US if Iceland makes him citizen: 

Japanese lawmaker

by Hiroshi Hiyama

ATTENTION - ADDS quotes from immigration official, details ///

</HEADLINE>

<DATELINE>

TOKYO, March 16

</DATELINE>

<TEXT>

<S Entail="28" s_id="0">

<REF YPE="PROPNAME">Bobby Fischer</REF>can escape US if Iceland makes 
+<REF ANT-ID="100" EXT="Bobby Fischer" ID="101">him</REF> citizen: 

Japanese lawmaker

by Hiroshi Hiyama

ATTENTION - ADDS quotes from immigration official, details ///

</S><S Entail="28-31" s_id="1">

Chess legend <REF  ID="102" YPE="PROPNAME">Bobby Fischer</REF>, who fa
+ces prison if <REF ANT-ID="102" EXT="Bobby Fischer" ID="103" YPE="PRO
+N">he</REF> returns to the United States, can only avoid deportation 
+from Japan if 

<REF ID="104" YPE="PROPNAME">Iceland</REF> upgrades <REF  ANT-ID="104"
+ EXT="Iceland's" ID="105">its</REF> granting of residency to full cit
+izenship, a Japanese lawmaker said Wednesday.

</S>
</TEXT>
</DOC>
[download]

and my expected file is mentioned as below .some of the REF tags do not have EXT ,for that cases there is not replacment

<DOC id="AFP_ENG_20050316.0102" type="story">

<HEADLINE>

Bobby Fischer can escape US if Iceland makes him citizen: 

Japanese lawmaker

by Hiroshi Hiyama

ATTENTION - ADDS quotes from immigration official, details ///

</HEADLINE>

<DATELINE>

TOKYO, March 16

</DATELINE>

<TEXT>

<S Entail="28" s_id="0">

<REF ID="100" YPE="PROPNAME">Bobby Fischer</REF>can escape US if Icela
+nd makes <REF  ANT-ID="100" EXT="Bobby Fischer" ID="101">Bobby Fische
+r</REF> citizen: 

Japanese lawmaker

by Hiroshi Hiyama

ATTENTION - ADDS quotes from immigration official, details ///

</S><S Entail="28-31" s_id="1">

Chess legend <REF ID="102" YPE="PROPNAME">Bobby Fischer</REF>, who fac
+es prison if <REF  ANT-ID="102" EXT="Bobby Fischer" ID="103" YPE="PRO
+N">Bobby Fischer</REF> returns to the United States, can only avoid d
+eportation from Japan if 

<REF ID="104" >Iceland</REF> upgrades <REF  ANT-ID="104" EXT="Iceland'
+s" ID="105">Iceland's</REF> granting of residency to full citizenship
+, a Japanese lawmaker said Wednesday.

</S>
</TEXT>
</DOC>
[download]

my code is

#!/bin/perl -w
use strict;
use XML::Twig;

 my $twig=new XML::Twig(twig_roots => {'TEXT' =>1} ,twig_handlers=>{'R
+EF' => \&REF}); # change the address of root to TEXT element because 
+REF is the children of TEXT
 my $field='REF';
$twig->parsefile("AFP_ENG_20050316.0102.xml"); #Parse the file
 my $root=$twig->root;
 my $rootchild=$root->children;

sub REF
{
 my ($twig ,$field)=@_;
 my $extension = $field->text;#keep the value of REF element
 my $att=$field->att('EXT');  #keep the value of Ext ( some REF elemen
+ts do not have EXT attribute) 
 print "$extension\n";
 $att->set_text($field);
 $extension=$att;
}
$twig->print;
[download]

I would like to know is there function in twig to remove the REF elements after this replacement but keep the value of them in sentence? thanks a lot.

I think this gets you a little closer:

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

my $twig = new XML::Twig(
    twig_handlers => { 'REF' => \&REF }
);
$twig->parsefile("AFP_ENG_20050316.0102.xml");    #Parse the file
$twig->print();

sub REF {
    my ( $twig, $field ) = @_;
    my $ref_text = $field->text();
    my $ref_ext  = $field->att('EXT');
    $field->set_text($ref_ext) if $ref_ext;
}
[download]

You can try to use the cut method to remove elements.

Thanks a lot dude. It is perfect and you are more :)