Note that your sub will cut off the input string if its length is larger than the padding width:

sub rpad { pack "A$_[1]", $_[0] };
my $string = 'averylongstringthatislongerthan10charsforsure!';

print "]", rpad($string, 10), "[\n";

__END__
]averylongs[
[download]

I am not sure if that is what the OP intended.