Reading the docs, I'd suspect that it would be as simple as this:

$ cat ex_NDBM_File_2.pl
use strict;
use warnings;
use Fcntl;
use NDBM_File;

tie(my %h, 'NDBM_File', 'ex_NDBM_File.ndbm', O_RDONLY, 0777) or die;

for my $k (sort keys %h) {
    print "$k --> '$h{$k}'\n";
}

$ perl ex_NDBM_File_2.pl
bar --> 'testing 12 .. hello, testing 12!'
foo --> '123 test'
[download]

I prepared the NDBM file like so:

$ cat ex_NDBM_File_1.pl
use strict;
use Fcntl;
use NDBM_File;

tie(my %h, 'NDBM_File', 'ex_NDBM_File.ndbm', O_RDWR|O_CREAT, 0777) or 
+die;

$h{foo}='123 test';
$h{bar}='testing 12 .. hello, testing 12!';

$ perl ex_NDBM_File_1.pl

$
[download]

...roboticus

When your only tool is a hammer, all problems look like your thumb.