Record the information as you parse the input:

use strict;
use warnings;
use Data::Dump;
my %volumes;

while (<DATA>) {
    next if ! /^(\w{1})\-(\d{3})\-(\d{1,2})\-(\d{1,2})\s+\((\d+)\)/;

    ++$volumes{$1}{-chapters} if ! exists $volumes{$1}{$2};
    ++$volumes{$1}{-sections} if ! exists $volumes{$1}{$2}{$3};
    ++$volumes{$1}{-items} if ! exists $volumes{$1}{$2}{$3}{$4};
    $volumes{$1}{$2}{$3}{$4} = $5;
}

print Data::Dump::dump (\%volumes);

__DATA__
B-001-01-01 (2)
B-001-01-02 (1)
B-001-02-01 (3)
B-001-02-02 (4)
B-001-02-03 (5)
B-002-01-01 (0)
[download]

Prints:

{
  B => {
         "-chapters" => 2,
         "-items" => 6,
         "-sections" => 3,
         "001" => {
               "01" => { "01" => 2, "02" => 1 },
               "02" => { "01" => 3, "02" => 4, "03" => 5 },
             },
         "002" => { "01" => { "01" => 0 } },
       },
}
[download]