7.2.1 AnyDBM_File - Provide Framework for Multiple DBMs

use AnyDBM_File;

This module is a "pure virtual base class" - it has nothing of its own. It's just there to inherit from the various DBM packages. By default it inherits from NDBM_File for compatibility with earlier versions of Perl. If it doesn't find NDBM_File, it looks for DB_File, GDBM_File, SDBM_File (which is always there - it comes with Perl), and finally ODBM_File.

Perl's dbmopen function (which now exists only for backward compatibility) actually just calls tie to bind a hash to AnyDBM_File. The effect is to bind the hash to one of the specific DBM classes that AnyDBM_File inherits from.

You can override the defaults and determine which class dbmopen will tie to. Do this by redefining @ISA:

@AnyDBM_File::ISA = qw(DB_File GDBM_File NDBM_File);

Note, however, that an explicit use takes priority over the ordering of @ISA, so that:

use GDBM_File;

will cause the next dbmopen to tie your hash to GDBM_File.

You can tie hash variables directly to the desired class yourself, without using dbmopen or AnyDBM_File. For example, by using multiple DBM implementations, you can copy a database from one format to another:

use Fcntl;         # for O_* values
use NDBM_File;
use DB_File;
tie %oldhash, "NDBM_File", $old_filename, O_RDWR;
tie %newhash, "DB_File",   $new_filename, O_RDWR|O_CREAT|O_EXCL, 0644;
while (($key,$val) = each %oldhash) {
    $newhash{$key} = $val;
}

http://www.perl.com/CPAN/src/misc/db.1.85.tar.gz
http://www.perl.com/CPAN/src/misc/gdbm-1.7.3.tar.gz