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

my $str = <<"XML";
<?xml version="1.0" encoding="utf-8"?>\r
<experiment title="Blood Pressure" version="1.2" softwareVersion="1.1">\r
<sourceFiles>\r
</sourceFiles>\r
<dataFiles>\r
<dataFile>cuff practice.dat</dataFile>\r
</dataFiles>\r
</experiment>\r
XML

# Troublesome code
print "******* Troublesome version\n";
my @fileList = qw(first second);
my $sub = bindArgs (\&insertFiles, \@fileList);
my $twig = XML::Twig->new (
    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 ();
}

##</code><code>##

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

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

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

<dataFiles>

<dataFile>cuff practice.dat</dataFile>

</dataFiles>

</experiment>