To get the derived statistics, loop over all the indices and count: 
my %table;

# The number of items of group $d of length N (where N is 1 to a
# max. length determined elsewhere) found in $b.

for my $file (keys %a) {
   for my $group (keys %{$a{$file}}) {
      for my $subgroup (keys %{$a{$file}{$group}}) {
         for my $length (@{$a{$file}{$group}{$subgroup}}) {
            $table{$file}{$subgroup}[$length]++;
         }
      }
   }
}

[download]
To print out a table, use another nested loop: 
my $file = 'xxx';
for my $length (1..$max_length) {
   for my $unique_d (keys %{$a{$file}{$group}}) {
      # printing stuff using $table{$file}{$unique_d}[$length]
   }
}

[download]
Update: fixed a typo in the second comment.

-Mark

In reply to Re: Extracting data from nested hashes by kvale
in thread Extracting data from nested hashes by knirirr

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.