use strict;
use warnings;
use XML::Twig;

my $twig = new XML::Twig (pretty_print => 'record');
my $root = new XML::Twig::Elt ('root');

$twig->set_root ($root);

my @pendingSections;

while (<DATA>) {
     chomp;
     
     if (/<h(\d+)>(.*)/i) {
          # deal with a header line
          # First pop any pending sections that need to be closed
          pop @pendingSections while @pendingSections > $1;

          while (@pendingSections < $1) {
               # Add section elements out to current header level
               my $level = @pendingSections + 1;
               my $elt = new XML::Twig::Elt ("section$level");
               
               if ($level == 1) {
                    $elt->paste (last_child => $root);
               } else {
                    $elt->paste (last_child => $pendingSections[-1]);
               }
               
               push @pendingSections, $elt;
          }
          
          my $headElt = new XML::Twig::Elt ("head", $2);
          
          $headElt->paste (last_child => $pendingSections[-1]);
          
     } else {
          # It's a paragraph
          if (@pendingSections) {
               $pendingSections[-1]->suffix ("$_\n");
          } else {
               $root->suffix ("$_\n");
          }
     }
}

$twig->print ();

__DATA__
<h1>Heading level 1
<h2>Heading level 2
<h3>Heading level 3
<h2>Heading level 2
<h1>Heading level 1
paragraph here
paragraph here