my $private_key = $sth->fetchrow(); #Stored as a string

#I need to put $private_key in a format readable by the function below

my $plaintext = $rsa->decrypt ( 
            Cyphertext => $cyphertext, 
            Key        => $private_key,
            Armour     => 1,
        ) || die $rsa->errstr();

##</code><code>##

$private_key = new Crypt::RSA::Key::Private (
                 Filename => 'private_key_file'
                );