As you probably know, Rijndael is a block-cipher algorithm. Your data must be a multiple of 16 in size. It seems like you know this. Keep in mind also that this is a binary algorithm -- there are no invalid values, either on input or output. One thing to look out for is encrypted data manipulation. Your encrypted data can contain anything. If you inadvertently perform any string-like operation on it, the results are unpredictable.

Don't use Crypt::Rijndael directly. Use it via Crypt::CBC.

And be sure to binmode file handles.

#!/usr/bin/perl
use warnings;
use strict;
use Crypt::Rijndael;

# keys must be 128, 192 or 256 "bits" long, 

# 8 bits in a letter, so 8 x 16 = 128
#my $key = 'abcdefgghjkloiuy'; #128 bits or 8 x 16
my $key = 'abcdefgghjkloiuyabcdefgghjkloiuy'; #256 bits or 8 x 32
print "key->$key\n";


my $plaintext= "adrqwqqqqqqqqqqqqqqqqqqqqqqwrxcq4gfq3g2q45g2q43g5";
print "plaintext->$plaintext\n";
my $plaintext16= get16($plaintext);
print "plaintext16->$plaintext16\n";


my $cbc = new Crypt::Rijndael $key, Crypt::Rijndael::MODE_CBC;
my $crypted = $cbc->encrypt($plaintext16);
print 'crypted->',"$crypted\n"; 
#keep encrypted string in different print string
#to avoid character corruption of preceding string


my $cbc1 = new Crypt::Rijndael $key, Crypt::Rijndael::MODE_CBC;
my $decrypted = $cbc1->decrypt($crypted);
print "decrypted->$decrypted\n";

#this sub makes all data blocksize of 16 bytes.
sub get16 {
    my $data = shift;
     return $data . "\0" x ( 16 - length($data)%16 );
      }
      
exit;

#You can also use this to get data that is in a multiple of 16 bytes:
# this requires $username < 16 bytes and prefix packs with spaces
#    sprintf '%16s', $username;
#or you can xor it
# $filecrypted = $cipher->encrypt($file^("\0"x16));

#To strip any padding after decryption:
#my $plaintext = $cipher->decrypt($secret_stuff);
#   $plaintext =~ s/^\0+//;  
# remove leading null bytes, use \s (not \0) for spaces
[download]

ENCRYPT SCRIPT
#!/usr/bin/perl
use DBI;
use Crypt::Rijndael;

$iv = 'same_16_character_string';
$cipher = Crypt::Rijndael->new( "a" x 32, Crypt::Rijndael::MODE_CBC );
$cipher->set_iv($iv);

$data = $cipher->encrypt($some_data);

** $data is then inserted into a blob field in mysql (data in the acco
+unts table)


DECRYPT SCRIPT
#!/usr/bin/perl
use DBI;
use Crypt::Rijndael;

$iv = 'same_16_character_string';
$cipher = Crypt::Rijndael->new( "a" x 32, Crypt::Rijndael::MODE_CBC );
$cipher->set_iv($iv);

$sql_data = "SELECT data FROM accounts WHERE account_id = '237' LIMIT 
+1";
$qry_data = $dbh->prepare($sql_data);
$qry_data->execute();
$data = $qry_data->fetchrow_hashref();
$data = $data->{data};
print $cipher->decrypt($data) . "\n\n";
[download]

Notice the first line of the documentation?

"Crypt::CBC compliant Rijndael encryption module"

That should tell you something!

use Crypt::CBC;
my $key = "a" x 32;
my $cipher = Crypt::CBC->new( -cipher => 'Rijndael', -key => $key );
my $ciphertext = $cipher->encrypt($plaintext);
[download]

use Crypt::CBC;
my $key = "a" x 32;
my $cipher = Crypt::CBC->new( -cipher => 'Rijndael', -key => $key );
my $plaintext  = $cipher->decrypt($ciphertext);
[download]

It handles padding, for you.
It handles salting for you.
It handles chaining for you.

And make sure your database field is large enough.

So close, and yet so far... You have left out the critical (and most likely problematic) piece; the DB insertion. I will bet that if you try to decrypt immediately after you encrypt, it will work fine. The problem is that either your insert or select is munging up the data. I have not used blobs with perl/dbi, so I am not aware of specific issues. Look at http://www.james.rcpt.to/programs/mysql/blob/. Is this the way you are creating the insert?

I can't imagine that's the problem, but I guess it's possible. Here's the update:

$sql_update = "UPDATE accounts SET data = '" . $data . "', encrypted =
+ '1' WHERE account_id = '" . $account_id . "'";
$qry_update = $dbh->prepare($sql_update);
$qry_update->execute();
[download]

$data is the encrypted piece