in reply to nested level section closing

Use a tool designed for the job. It looks like you may be generating XML, perhaps by parsing some lightly marked up, sorta HTML, source. So lets go with that for a moment and see what can be done:

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

[download]

Prints:

<root>
  <section1>
    <head>Heading level 1</head>
    <section2>
      <head>Heading level 2</head>
      <section3>
        <head>Heading level 3</head>
      </section3>
      <head>Heading level 2</head>
    </section2>
    <head>Heading level 1</head>paragraph here
paragraph here
</section1>
</root>

[download]

which isn't quite what you showed (especially the root element), but does do the parsing/rendering magic it seems you may be after.

DWIM is Perl's answer to Gödel

In Section Seekers of Perl Wisdom