You can apply the usual untainting mantra by also using a BEGIN section:

use strict;
use warnings;
use FindBin 1.51 qw( $RealBin );
BEGIN {
    ($RealBin) = $RealBin =~ /(.+)/;
}
use lib "$RealBin/../lib";

use MyLib;
[download]

Of course, you need to trust your installation to not mess with the current working directory (because this is why $RealBin comes out tainted in the first place).