use strict;
use XML::Twig;
 
my $string =<<EOXML;
<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>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
<a>here the paragraph comes</a>
</xml>
EOXML
 
my $seen_a = 0;
my $twig = new XML::Twig(
                
      twig_handlers => {
                                 p => sub { $_->add_id() },
                                 a => sub { $_->set_id_seed('sec') unless $seen_a++ ;$_->add_id() },
                
                               },
      pretty_print => 'indented'
);
 
$twig->set_id_seed( "fig" );
$twig->parse($string);
print $twig->sprint;