use strict;
use XML::Twig;

my $string = '<xml>
<p>here the paragraph comes</p>
<p>here the paragraph comes</p>
<p>here the paragraph comes</p>
</xml>';

my $twig = new XML::Twig(
					
		twig_handlers => {
	
           p => sub { $_->add_id() }, #insert element id

                               },
		pretty_print => 'indented' #print format
);

$twig->set_id_seed( '' );

$twig->parse($string);

$string = $twig->sprint;

print $string;

output:
-------
<xml>
  <p id="1">here the paragraph comes</p>
  <p id="2">here the paragraph comes</p>
  <p id="3">here the paragraph comes</p>
</xml>