Yeah but in your original "required output" you show two elements with the same value for 'id', so forgive me for being a little confused.

Update: right got you. You can set the id seed in the appropriate handler, you will need some extra logic if the 'a' and 'p' elements are intermixed but basically you can do something like:

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') unl
+ess $seen_a++ ;$_->add_id() },
                
                               },
      pretty_print => 'indented'
);
 
$twig->set_id_seed( "fig" );
$twig->parse($string);
print $twig->sprint;
[download]

/J\

You can (ab)use the magical increment properties of ++:

my $c='aff001';
print $c++, "\n" for 0..5;
__END__
aff001
aff002
aff003
aff004
aff005
aff006
[download]

You should be careful with overflows, though:

my $c='aff998';
print $c++, "\n" for 0..5;
__END__
aff998
aff999
afg000
afg001
afg002
afg003
[download]