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>
<p>here the paragraph comes</p>
</xml>';

my $i =1;

$string =~ s/<p>/'<p id="'.$i++.'">'/egs;

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>
<p id="4">here the paragraph comes</p>
</xml>