#!/usr/bin/perl -w

require 5.6.0;
use strict;
use Data::Dumper;

our @input = ({ class => "Chemistry", department => "v6", count => 18 },
              { class => "German I", department => "v6", count => 27},
              { class => "French II", department => "h4", count => 9 });


our %collate;

foreach (map { [ $_->{department}, $_ ] } @input) {
  push(@{$collate{$$_[0]}}, $$_[1]);
}

our @output = map { { department => $_, classes => $collate{$_} } } keys %colla\
te;

print Data::Dumper->new([\@output], [qw|*output|])->Dumpxs;

##</code><code>##

foreach (@input) {
 push(@{$collate{$_->{department}}}, $_);
}