Memoize problem with old Perl

[Anonymous Monk]

#!/usr/bin/perl
# SSCCE from examples at "perldoc Memoize"
# Memoize works as expected on recent Perls 5.20-28.
# Builds huge cache and does not work on 5.18.
# Thoughts on why, and how to handle it?

use strict;
use warnings;
use diagnostics;

use Data::Dumper;
use DB_File;
use File::Spec;
use integer;
use Memoize;
use POSIX ();

my $title = 'Compute Fibonacci numbers';
my $tempd = File::Spec->tmpdir;
my $tempf = $] . '_memoize_fib';
my $cache = File::Spec->catfile($tempd,$tempf);

my $ARGS = @ARGV ? shift : 8;

unlink $cache if -e $cache and $ARGS eq 'purge';

my $db = tie my %cache => 'DB_File', $cache, POSIX::O_RDWR|POSIX::O_CR
+EAT, 0666;
memoize 'fib', SCALAR_CACHE => [HASH => \%cache]; 

print "Perl $]
Memoize  $Memoize::VERSION
Tempdir  $tempd
Tempfile $tempf
Filesize ", -s $cache, " bytes
Question $ARGS
Answer ", fib($ARGS), "\n\n",

# Because Data::Dumper panics on multigig dumps with
# panic: sv_setpvn called with negative strlen -8260

eval { Data::Dumper->Dump([\%cache],[('%cache')]) };
unlink $cache if $@;

print "$title\nUsage: $0 n, $0 purge\n";

sub fib {
  my $n = shift;
  return $n if $n < 2;
  fib($n-1) + fib($n-2);
}

Perl 5.018002
Memoize  1.03
Tempdir  /tmp
Tempfile 5.018002_memoize_fib
Filesize 146800640 bytes
Question 8
Answer 21

$%cache = {
            '0' => undef,
            '1' => undef,
            '2' => undef,
            '3' => undef,
            '4' => undef,
            '5' => undef,
            '6' => undef,
            '7' => undef
          };

Perl 5.020003
Memoize  1.03
Tempdir  /tmp
Tempfile 5.020003_memoize_fib
Filesize 49152 bytes
Question 8
Answer 21

$%cache = {
            '0' => '0',
            '2' => '1',
            '4' => '3',
            '6' => '8',
            '1' => '1',
            '3' => '2',
            '5' => '5',
            '7' => '13'
          };

Perl 5.028000
Memoize  1.03_01
Tempdir  /tmp
Tempfile 5.028000_memoize_fib
Filesize 49152 bytes
Question 8
Answer 21

$%cache = {
            '0' => '0',
            '2' => '1',
            '4' => '3',
            '6' => '8',
            '1' => '1',
            '3' => '2',
            '5' => '5',
            '7' => '13'
          };

[choroba]

Weird, works for me:

Perl 5.018002
Memoize  1.03
Tempdir  /tmp
Tempfile 5.018002_memoize_fib
Filesize 12288 bytes
Question 8
Answer 21

$%cache = {
            '0' => '0',
            '2' => '1',
            '4' => '3',
            '6' => '8',
            '1' => '1',
            '3' => '2',
            '5' => '5',
            '7' => '13'
          };