Some very, very minor points.

You might put the definition of  $username, $password, $dsn at the very top of the script. This will make it easier to make simple modifications quickly. Something like:

use strict;
use DBI;
use Getopt::Std;

my $dsn = "DBI:mysql:$database:localhost";
my $username = 'username';
my $passwd = unpack("u", $enc_passwd);

######### No User servicable parts below ############
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print unpack("u", $enc_passwd);
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