how to copy child or tag in xml twig by xpath?

Question

You seem to be having problems with scalars vs arrays. Maybe a consequence of using jQuery in a previous life? ;--)

In any case, your sub would work if you write this:

sub Tag_Alt{
  my ($Tag_move, $tagm) = @_;
  my @c = $tagm->findnodes('//front/sample/');   
  my @children= map { $_->cut_children } @c;
  foreach my $child (@children) {
    $child->paste('first_child', ($tagm->findnodes('//body/'))[0]); 
  }     
}

I would write this differently though:

#!/usr/bin/perl

use strict;
use warnings;

use XML::Twig;

my @samples;

my $Tag_move = XML::Twig->new(
    twig_handlers =>{ # cut and store the everything within sample
                      sample => sub { push @samples, $_->cut_children; },
                      # paste the samples within the body
                      body   => sub { foreach my $sample (@samples) 
                                        { $sample->paste( first_child => $_); }
                                    },
                    },
    pretty_print => 'indented',
    empty_tags => 'expand',   # otherwise the empty sample is output as <sample/>
);
$Tag_move->parsefile( "input.xml")
        ->print;

BTW, do yourself a favour, and use strict and warnings. This will catch a lot of the mistakes you make. You will get errors instead of allowing unknown variables to be created silently.