Try this:

foreach my $key (
    sort {
        $self->{_counts}->{$a} <=> $self->{_counts}->{$b}
    } keys %{ $self->{_counts} }
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If you do $self->{_counts}{$b}, Perl thinks $self->{_counts} is a real Hash, not an HashRef. If you use the arrow to dereference, it is easier to read.