--- in FOO/Bar/Baz.pm

my %function_table;

sub set_callback {
	my ($key, $function) = @_;
	$function_table{$key} = $function;
}

sub some_function {
	if (defined $function_table{'login'}) {
		 $function_table{'login'}->();
	}
	else {
		die "No callback declared for login";
	}
}

--- in FOO/Bar.pl

use FOO::Bar;

sub login { }

FOO::Bar->set_callback('login', \&login);