In Data::Dumper's output, bless signifies a blessed reference. Have a look at this example:

use strict;
use warnings;
use DateTime;
use Data::Dumper;
my $t = DateTime->now;
print Dumper $t;
[download]

which creates this output:

$VAR1 = bless( {
                 'local_rd_secs' => 25639,
                 'local_rd_days' => 735072,
                 'rd_nanosecs' => 0,
                 'locale' => bless( {
                                      'default_time_format_length' => 
+'medium',
                                      'native_territory' => 'United St
+ates',
                                      'native_language' => 'English',
                                      'native_complete_name' => 'Engli
+sh United States',
                                      'en_language' => 'English',
                                      'id' => 'en_US',
                                      'default_date_format_length' => 
+'medium',
                                      'en_complete_name' => 'English U
+nited States',
                                      'en_territory' => 'United States
+'
                                    }, 'DateTime::Locale::en_US' ),
                 'local_c' => {
                                'hour' => 7,
                                'second' => 19,
                                'month' => 7,
                                'quarter' => 3,
                                'day_of_year' => 204,
                                'day_of_quarter' => 23,
                                'minute' => 7,
                                'day' => 23,
                                'day_of_week' => 2,
                                'year' => 2013
                              },
                 'utc_rd_secs' => 25639,
                 'formatter' => undef,
                 'tz' => bless( {
                                  'name' => 'UTC'
                                }, 'DateTime::TimeZone::UTC' ),
                 'utc_year' => 2014,
                 'utc_rd_days' => 735072,
                 'offset_modifier' => 0
               }, 'DateTime' );
[download]

This tells you that $t is a blessed hash reference with classname DateTime. The hash itself contains all the members of the class some of which again are blessed hash references (like locale and tz) of different types (classes).

In the structure you have posted, it seems that the class BLOCK has elements that are blocks themselves, like BLOCKs of BLOCKs. In a way this is not much different from arrays of arrays.