The following may be a useful starting point for you:

use strict;
use warnings;

my @sectionCount = (1, 1);

while (<DATA>)
  {
  chomp;

  if (/<\/sec>/)
    {
    pop @sectionCount;
    ++$sectionCount [-1];
    next;
    }

  next if ! /<sec id=/;

  push @sectionCount, 1;

  my ($secNum) = /<no>(.*?)<\/no>/;
  print "Section nesting error in or before $secNum\n"
    if $secNum ne join ".", @sectionCount[0..(@sectionCount-2)];
  }

[download]
__DATA__
<sec id="00005"><no>1.1</no>
    <sec id="00010"><no>1.1.1</no>
    </sec>
    <sec id="00015"><no>1.1.2</no>
        <sec id="00020"><no>1.1.2.1</no>
        </sec>
        <sec id="00025"><no>1.1.2.2</no>
        </sec>
    </sec>
    <sec id="00015"><no>1.1.3</no>
    </sec>
</sec>
<sec id="00005"><no>1.2</no>
    <sec id="00010"><no>1.2.1</no>
    </sec>
    <sec id="00015"><no>1.2.2</no>
        </sec>
        <sec id="00020"><no>1.2.2.1</no>
        </sec>
        <sec id="00025"><no>1.2.2.2</no>
    </sec>
    <sec id="00015"><no>1.2.3</no>
    </sec>
</sec>

Section nesting error in or before 1.2.2.1
Section nesting error in or before 1.2.2.2
Section nesting error in or before 1.2.3

[download]
Perl is Huffman encoded by design.

In reply to Re: section nesting by GrandFather
in thread section nesting by anniyan

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.