here is one way, good for small data sets

#!/usr/bin/perl -- 

use strict;
use warnings;


use XML::Simple;
use Data::Dumper;

my $xml = q~<?xml version='1.0'?>
<param>
    <product_name>ABC</product_name>
    <lcns>
      <lcns1>
          <ext>1000</ext>
          <lcns>text1</lcns>
      </lcns1>
      <lcns2>
          <ext>40</ext>
          <lcns>text2</lcns>
      </lcns2>
      <lcns3>
          <lcns>text3</lcns>
      </lcns3>
    </lcns>
</param>~;

print $xml,$/;
print Dumper( XMLin($xml));

# my $VAR1 = XMLin($xml);
my 
$VAR1 = {
          'lcns' => {
                    'lcns3' => {
                               'lcns' => 'text3'
                             },
                    'lcns2' => {
                               'ext' => '40',
                               'lcns' => 'text2'
                             },
                    'lcns1' => {
                               'ext' => '1000',
                               'lcns' => 'text1'
                             }
                  },
          'product_name' => 'ABC'
        };
# insert lcns4
$VAR1->{lcns}{lcns4} = { qw~ ext 10 lcns text4 ~ };
# edit existing entry lcns2
$VAR1->{lcns}{lcns2}{ext} = 50;
$VAR1->{lcns}{lcns2}{lcns} = "new text2";
## edit existing entry lcns2
$VAR1->{lcns}{lcns2} = { qw~ ext 50 lcns new text 2~ };

print XMLout($VAR1, NoAttr=>1,RootName=>'param',XMLDecl => 1, keyattr 
+=> []);
__END__
<?xml version='1.0'?>
<param>
    <product_name>ABC</product_name>
    <lcns>
      <lcns1>
          <ext>1000</ext>
          <lcns>text1</lcns>
      </lcns1>
      <lcns2>
          <ext>40</ext>
          <lcns>text2</lcns>
      </lcns2>
      <lcns3>
          <lcns>text3</lcns>
      </lcns3>
    </lcns>
</param>
$VAR1 = {
          'lcns' => {
                    'lcns3' => {
                               'lcns' => 'text3'
                             },
                    'lcns2' => {
                               'ext' => '40',
                               'lcns' => 'text2'
                             },
                    'lcns1' => {
                               'ext' => '1000',
                               'lcns' => 'text1'
                             }
                  },
          'product_name' => 'ABC'
        };
<?xml version='1.0' standalone='yes'?>
<param>
  <lcns>
    <lcns1>
      <ext>1000</ext>
      <lcns>text1</lcns>
    </lcns1>
    <lcns2>
      <ext>50</ext>
      <lcns>new</lcns>
      <text>2</text>
    </lcns2>
    <lcns3>
      <lcns>text3</lcns>
    </lcns3>
    <lcns4>
      <ext>10</ext>
      <lcns>text4</lcns>
    </lcns4>
  </lcns>
  <product_name>ABC</product_name>
</param>
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