I want to perform something like a:
  SELECT department, count(department)
  FROM table
  GROUP BY department

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.. but on a Perl hash.

Let's say I have something like the following: 

use strict;

my %testhash = ( 8172263 => { name => 'Robert',
                              department => 'music' },
                 8177234 => { name => 'Elizabeth',
                              department => 'chemistry' },
                 7226331 => { name => 'Reginald',
                              department => 'chemistry' },
                 8117223 => { name => 'Ralph',
                              position => 'King' },
                 9118223 => { name => 'Richard',
                              department => 'music' },
                 9119233 => { name => 'Pamela',
                              department => 'music' }
                 );

# how do I discover the number of 
#   chemistry students in my hash very quickly?

my $musicstudents = grep { $testhash{$_}->{department} &&
                           $testhash{$_}->{department} =~ m/music/
                         }
                         keys %testhash;
print( "$musicstudents\n" );

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But that's when I know there are music students before hand. How do I build up a hash very quickly containing item counts within a column?

e.g. if I called some (inline maybe) function

   my %outputhash = somefunction(\%testhash, 'department');

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the output hash would look something like:
  # my %outputhash = ( music => 3,
  #                    chemistry => 2 );

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What I'm looking for is code efficiency here, as I could build this up using lots of loops but would rather not do that.. maybe something fancy with a map is possible but I just can't see it for the moment.

Update: Thanks to Zaxo and ysth for the answer. The precise variant I will use in my code is:
  my %outputhash;
  $outputhash{$_->{department}}++ for grep { $_->{department}
                                            } values %testhash;

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Embaressingly enough I knew about the keys function but had been unaware of the values function until now.. *picks up Camel book again*

In reply to Simulating a summary of a hash by monarch

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