in file FOO/Bar/Baz.pm

package FOO::Bar::Baz;

sub some_function {
	login();
}

in file FOO/Bar.pl

package FOO::Bar;

sub login {
	# something interesting
}


##</code><code>##


--- in the new module FOO/Bar.pm

package FOO::Bar

sub login { }

--- in FOO/Bar/Baz.pm

package FOO::Bar::Baz
use FOO::Bar;

sub some_function {
	FOO::Bar::login();
}

--- in the script
(remove the login function and proceed as normal)


##</code><code>##


--- 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);