Thanks for these answers, it runs well with:

use Cwd;
$ARGV[0] = cwd()."/$ARGV[0]" unless $ARGV[0] =~ /^(\/.*)$/ ; # Abs.
use FindBin qw($RealBin);
chdir "$RealBin/..";
use lib "$RealBin";
[download]

and some minimal file check.