Firstly, your sample XML isn't valid XML as it has no root node so I wouldn't expect it to parse at all. I've changed it to this:

<doc>
<para>aaaaaaaaaaaaaaaaa  dsnbbtejk</para>
<para>aldmflskddddddddddddddddddffffffffffffff</para>
</doc>
[download]

Secondly, you seem a little confused about which elements you are looking for. At one point you look for elements called 'doc', but there aren't any elements called that in your sample data.

I think you want something more like this:

use strict;
use warnings;

use XML::Twig;
my $file = 'test.xml';

my $tmp;
$tmp = XML::Twig->new();
$tmp->parsefile($file);
my $root = $tmp->root;

foreach my $product ($root->children('para')){
  my $para = $product->first_child_text;
  print "$para\n";
}
[download]

<parent>
 <child>
  <second_child></second_child>
 </child>
</parent>
[download]

As others have pointed out your (not quite) XML data is not consistent with your code. The following sample may help:

use strict;
use warnings;

use XML::Twig;
my $xml = <<XML;
<root>
<para>aaaaaaaaaaaaaaaaa  dsnbbtejk</para>
<para>aldmflskddddddddddddddddddffffffffffffff</para>
</root>
XML

my $tmp = XML::Twig->new();
$tmp->parse ($xml);
my $root = $tmp->root;

print $_->trimmed_text (), "\n" for $root->children ('para');
[download]

Prints:

aaaaaaaaaaaaaaaaa dsnbbtejk
aldmflskddddddddddddddddddffffffffffffff
[download]