Perhaps you could look at the BerkeleyDB CPAN module.

This is an interface to BerkeleyDB (2.x or 3.x) that uses the native API instead of the DB1.85 API used by DB_File.

The reason I think it's relevant is that you can use a "Recno" format database, and tie it to an array. (However, the documenation is unclear on this, and I haven't got the module installed to hand).

Here is an extract from an example in the documentation:

use strict;
use BerkeleyDB;
my $filename = "text" ;
unlink $filename ;
my @h ;
tie @h, 'BerkeleyDB::Recno',
 -filename   => $filename,
 -Flags      => DB_CREATE,
 -Property   => DB_RENUMBER
   or die "Cannot open $filename: $!\n" ;


# Add a few key/value pairs to the file
$h[0] = "orange" ;
$h[1] = "blue" ;
$h[2] = "yellow" ;
push @h, "green", "black" ;
[download]

The problem then becomes if you can actually use MLDBM to tie an array instead of a hash, and TBH I don't know.

But I would have thought it would be fairly easy to add such functionality to MLDBM if it isn't there.