I can't eliminate the need for a temporary hash: having attempted it, I'm inclined to think it's not a nice thing to do, since you always seem to end up needing to collate as a side-effect of iterating over the input... Anyway, here's what I got:

#!/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 => $collate{$_} } } ke
+ys %colla\
te;

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

(Subquestion: is that constuct I used in the foreach() what's called a Schwartian transform?)

Update:

(Subanswer: It isn't)

Clearly I'm insufficiently awake. %collate can be built like this:

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

The transform is redundant.

Update II

tilly suggested I point out that the code doesn't work on older Perls, even 5.005.

Afaict, the changes for perl 5.005 would be:

• replace "require 5.6.0" to the appropriate version
• either change "our $foo" into "my $foo", or separate the declaration and assignments, and declare them in a "use vars qw($foo)" construct.

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