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

# Generate the XML file
my $str = do {local $/; <DATA>};
open OUT, '>', 'temp.xml';
print OUT $str;
close OUT;

# Troublesome code
print "******* Troublesome version\n";
my @fileList = qw(first second third);
my $sub = bindArgs (\&insertFiles, \@fileList);
my $twig = XML::Twig->new (
    discard_spaces => 1,
    pretty_print => 'indented',
    twig_roots => {'sourceFiles' => $sub,}, # Insert file list
    twig_print_outside_roots => 1, # Print the rest
    );

$twig->parsefile ('temp.xml');

# Ok code
print "******* Ok version\n";
$twig = XML::Twig->new (
    discard_spaces => 1,
    pretty_print => 'indented',
    twig_roots => {'sourceFiles' => $sub,}, # Insert file list
    twig_print_outside_roots => 1, # Print the rest
    );

$twig->parse ($str);


sub bindArgs {
  my ($sub, @args) = @_;
  return sub {$sub->(@args, @_);};
}


sub insertFiles {
    my ($sourceFiles, $twig, $node) = @_;
    
    for (@$sourceFiles) {
        my $eFile = $twig->root->new (sourcefile => $_); # create the element
        $eFile->paste (first_child => $node);
    }
    $node->print ();
}

__DATA__
<?xml version="1.0" encoding="utf-8"?>
<experiment title="Blood Pressure" version="1.2" softwareVersion="1.1">
   <sourceFiles>
   </sourceFiles>
   <dataFiles>
      <dataFile>arm_pulse.dat</dataFile>
      <dataFile>bp_Ex1.dat</dataFile>
      <dataFile>bp_pulse.dat</dataFile>
      <dataFile>cuff practice.dat</dataFile>
   </dataFiles>
</experiment>

##</code><code>##

******* Troublesome version
<?xml version="1.0" encoding="utf-8"?>

<experiment title="Blood Pressure" version="1.2" softwareVersion="1.1">

   
  <sourceFiles>
    <sourcefile>third</sourcefile>
    <sourcefile>second</sourcefile>
    <sourcefile>first</sourcefile>
  </sourceFiles>

   <dataFiles>

      <dataFile>arm_pulse.dat</dataFile>

      <dataFile>bp_Ex1.dat</dataFile>

      <dataFile>bp_pulse.dat</dataFile>

      <dataFile>cuff practice.dat</dataFile>

   </dataFiles>

</experiment>

******* Ok version
<?xml version="1.0" encoding="utf-8"?>
<experiment title="Blood Pressure" version="1.2" softwareVersion="1.1">
   
  <sourceFiles>
    <sourcefile>third</sourcefile>
    <sourcefile>second</sourcefile>
    <sourcefile>first</sourcefile>
  </sourceFiles>
   <dataFiles>
      <dataFile>arm_pulse.dat</dataFile>
      <dataFile>bp_Ex1.dat</dataFile>
      <dataFile>bp_pulse.dat</dataFile>
      <dataFile>cuff practice.dat</dataFile>
   </dataFiles>
</experiment>