Try changing around the position of where you declare the Hello package.

package Hello;

sub printit;

# NB: used $bar instead of special var $b for sanity
our $bar = "Hello, world!\n";

sub printit {
  print $bar;
}

1;

package main;

use strict;
use warnings;

&Hello::printit();

__output__

Hello, world!
[download]

It now works because $bar has been declared, so &printit() has something to print.
HTH

broquaint