I'm not really sure what you're trying to do, but I'm assuming that it will be easier for you if you can read the blocks into a data structure. This code reads the blocks into a hash of arrays. The keys of the hash are 'A' and 'B' and the blocks are put into the appropriate spot in the arrays depending on their id.

use strict;
use warnings;

use Data::Dumper;

my %blocks;

local $/ = '';

while (<DATA>) {
  my ($type) = /block_type=(A|B)/;
  my ($id)   = /block_id=(\d+)/;

  unless ($type && $id) {
    warn "Invalid block syntax in record $.\n$_\n";
    next;
  }

  $blocks{$type}[$id] = $_;
}

print Dumper \%blocks;

__DATA__
# A type A block
block_type=A
block_id=1
magic=true
unlikeliness=2

# Another type A block
block_type=A
block_id=2
magic=false
unlikeliness=9

# A type B block
block_type=B
block_id=1
colour=green
warty=false
horns=1

# Another type B block
block_type=B
block_id=2
colour=red
warty=false
horns=7

# A further type B block
block_type=B
block_id=3
colour=red
warty=true
horns=1
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