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add_id 

...
set_id_seed ($prefix) 

by default the id generated by add_id is twig_id_<nnnn>, set_id_seed c
+hanges the prefix to $prefix and resets the number to 1

use strict;
use XML::Twig;
...
  <p id="2">here the paragraph comes</p>
  <p id="3">here the paragraph comes</p>
</xml>