Why not simply define an static attribute registry in the class (or package if you wish) Queue? You could specify which registry to use by setting it via a class method setRegistry in class Queue.

package Queue;
my $register;

sub new {...}
sub setRegister {
  my ($pkg, $reg) = @_;
  $register = $reg;
}
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and in main.pl:

my $reg = Register->new();
my $queue = Queue->new();
Queue->setRegister($reg);
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This is the simplest approach just to give you the idea, and not how I'd actually do it, but I can elaborate if there's interest.

Hope this helps, -gjb-