my %cds = (
  classical => [
    [ 'Mozart', 'Requiem' ],
    [ 'Beethoven', '3rd Symphony' ]
  ],
  rock => [
    [ 'Van Halen', '1984' ]
  ],
  jazz => [
    [ 'Miles Davis', 'Birth of the Cool' ],
    [ 'Stan Getz', 'Brothers' ]
  ],
);
for( sort keys %cds ) {
  print $_, ": ", scalar @{ $cds{$_} }, " in group\n";
  for( @{ $cds{$_} } ) {
    print "\t", join(",", @{ $_ }), "\n";
  }
}