#!/usr/bin/perl

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

my $twig = new XML::Twig(
    twig_handlers => {
        '/article/fig' => sub {
            my $label = $_->first_child('label') or return;
            my $caption = $_->first_child('caption') or return;
            $label->move('first_child', $caption);
            $_->flush;
        }
},
    pretty_print => 'indented',                                     
);

$twig->parse(<<'XML');
<?xml version="1.0"?>
<article>
<label>here label for testing</label>
<fig id="fig001" position="float"><label>Fig. 1.</label><caption><p>first</p></caption></fig>
<fig id="fig002" position="float"><label>Fig. 2.</label><caption><p>second</p></caption></fig>
<fig id="fig003" position="float"><label>Fig. 3.</label><caption><p>third</p></caption></fig>
<fig id="fig004" position="float"><label>Fig. 4.</label><caption><p>fourth</p></caption></fig>
<fig id="fig005" position="float"><label>Fig. 5.</label><caption><p>fifth</p></caption></fig>
<fig id="fig006" position="float"><label>Fig. 6.</label><caption><p>sixth</p></caption></fig>
</article>
XML

$twig->print;