#!/usr/bin/perl

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

my $twig = new XML::Twig(                   
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

my @labels = $twig->get_xpath('/article/fig/label'); 
my @captions = $twig->get_xpath('/article/fig/caption');
for my $i (0..$#labels) {
    $labels[$i]->move('first_child', $captions[$i]);
}

$twig->print;