More generic than haukex's, more pure-perl than Athanasius's, more functional than OO

Shorter than huck's ;-P

sub count {
    my ($data, $fields) = @_;
    $fields = [$fields] unless ref $fields;
    my $count = {};
    for my $obj (@$data) {
        my $ref = $count;
        $ref = $ref->{ $obj->{ $fields->[$_] } } //= {}
            for 0..$#$fields-1;
        $ref->{ $obj->{ $fields->[-1] } }++;
    }
    return $count;
}
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(For input, output and other context see my other node)