I leave the file stuff up to you, since that part is easy and you have to code at least some of the project. But here's the regex you need:

use strict;
use warnings;

my %results;

$_ = join '', <DATA>;

($results{'no_of_items'}) = m/Number of Items\s+:\s+(\d+)/;

($results{'size_of_store'}) = m/Size of the Store\s+:\s+(\d+)/;

($results{'aa'}, $results{'ba'}, $results{'ab'}, $results{'bb'}) = m/\
+s+a\s+b\s+<-- classified as\s+(\d+)\s+(\d+)\s+\|\s+\w+ = \d+\s+(\d+)\
+s+(\d+)/;

### Just to display what's in hash, you still need to output it in you
+r format
use Data::Dumper;
print Dumper(\%results);

__DATA__
start of tree
------------------

Da         
|  something > 1234: 1 (81.06/25.89)

Number of Items  :     15

Size of the Store :     30

something else here

=== Summary Matrix ===

     a     b   <-- classified as
 11111   222 |     a = 0
  3333   444 |     b = 1
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