Now that you have DB_File installed, you could also test what happens when you open/tie your 5-yr-old data file as a plain-vanilla DB_File hash, just to make sure it's still readable, and to get a look at what the keys and values are.

UPDATE: In case it helps, here's a quick/easy way to dump out the contents of a DB_File hash:

use strict;
use DB_File;

my $Usage = "Usage: $0 db_file_name\n";
die $Usage unless ( @ARGV == 1 and -f $ARGV[0] );

my %file_hash;
tie ( %file_hash, 'DB_File', $ARGV[0] )
    or die "DB_File: $ARGV[0]: $!";

while ( my ( $key, $val ) = each %file_hash ) {
    print "key: $key\n$val\n;\n";
}
untie %file_hash;
[download]

(another update: this assumes that your key strings do not contain line-breaks while you values might; in any case, you could also transliterate things to suit your taste -- e.g.  s/(\p{IsCntrl})/sprintf("0x{%02x}",ord($1))/ge; or something like that.)

The code:

use strict;
use MLDBM qw(DB_File);
my $db = 'data_db';
my %records;
my $dbh = tie %records, 'MLDBM', $db or die $!;
my $junk = 1;
[download]

I wasn't able to replicate your results. I ran the snippet provided on the MLDBM man page, adding "qw(DB_File)" as needed, and used the resulting hash file as input for your snippet -- no errors. When I added "use Data::Dumper" and "print Dumper(...)" (as in the man page code), the output was as expected.

Maybe you should check that your MLDBM and Data::Dumper modules are both up-to-date?

So the bottom line is that once I installed DB_File it was working, but for some reason the debugger couldn't (and still can't) look at the tied hash.