Here is how I usually do it:

1. Create a separate module with all the shared staff:

package Common;

use constant GLOB_DEBUG_MODE = 1;
use constant GLOB_SITE_NAME = 'My site';

1;
[download]

2. use Common; from anywhere that I need. For example:

package Main;

use Common;

sub print_message {
  my $msg = shift;
  # Use default site name if no message specified.
  unless ($msg) { $msg = Common::GLOB_SITE_NAME; }
  # Prepend with "DEBUG: " if in debug mode.
  $msg = "DEBUG: $msg" if (Common::GLOB_DEBUG_MODE);
  
  print $msg;
}

1;
[download]

Update: I've answered a bit a wrong question. :) You were asking about global variables, while I described constants. :)

Update: Fixed two misplaced 'Common's. :)