Error deleting DBM::Deep file (ref count?)

[bobf]

I stumbled across a bug in my code that I tracked down to something I am doing with a DBM::Deep file. When I assign the inner key of a DBM hash-of-hrefs to another variable and then try to delete the file, I get an error when the inner key is a href but not when it is a non-ref scalar.

use strict;
use warnings;

use DBM::Deep;

my $db_file = 'href_nocopy';

{
    my $db = DBM::Deep->new( $db_file );
    $db->{key} = {};
}

print "\nDeleting $db_file\n";
unlink( $db_file ) || warn $!;  # succeeds

$db_file = 'href_copy';

{
    my $db = DBM::Deep->new( $db_file );
    $db->{key} = {};
    my $db2 = $db->{key};
}

print "\nDeleting $db_file\n";
unlink( $db_file ) || warn $!;  # fails (permission denied)

$db_file = 'nonref_copy';

{
    my $db = DBM::Deep->new( $db_file );
    $db->{key} = 1;
    my $db2 = $db->{key};
}

print "\nDeleting $db_file\n";
unlink( $db_file ) || warn $!; # succeeds

Any ideas? I didn't find any documentation about this elsewhere.

Thanks