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)];
  }