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)];
  }

##</code><code>##

__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