use strict;

my $count = 1;
my $line  = 0;

while (<DATA>)
{
  $line++;

    $count = 1, next
        if /^<main>/;
    
    die "out of order at line $line!" if
        /<sub(\d+)>/ && $1 != $count++;
}

__DATA__
<main>Ability-oriented motivation, 00206.p0105
<main>Ability test
<sub1>in conjunction with achievement batteries, 00226.p0085
<sub3>definition, 00226.g0005
<main>Aboriginal athletes, 00954.p0155
<sub2>Absolutism, 00688.p0010