I would use hashes of hashes and then count the number of entries. This is just a simple flat-file to tree generation question, akin to flatfile-to-xml constructor.

#!/usr/bin/perl

print "Hello, World...\n";
use strict;
use Data::Dumper;
my @in = qw/
fruit:apple:cox
fruit:apple:pippin
fruit:apple:granny
fruit:banana:yellow
fruit:banana:green
/;

#make an anonymous hash
my $h = {};

foreach  (@in) {
    my @a = split ':',$_;
    my $branch = shift @a;
    my $species =  shift @a;
    my $breed = shift @a;
    $h->{$branch}->{$species}->{$breed} = $h->{$branch}->{$species}->{
+$breed} + 1 || 1;
}

print Dumper($h);

print "there are ".scalar (keys %{$h->{fruit}})." fruit species\n";
print "there are ".scalar (keys %{$h->{fruit}->{apple}})." apple breed
+s\n";
print "there are ".scalar (keys %{$h->{fruit}->{banana}})." banana bre
+eds\n";

print "Good luck with your homework\n";
[download]

C:\>perl test.pl
Hello, World...
$VAR1 = {
          'fruit' => {
                       'apple' => {
                                    'pippin' => 1,
                                    'cox' => 1,
                                    'granny' => 1
                                  },
                       'banana' => {
                                     'green' => 1,
                                     'yellow' => 1
                                   }
                     }
        };
there are 2 fruit species
there are 3 apple breeds
there are 2 banana breeds
Good luck with your homework
[download]