Why am I getting "Use of uninitialized value in hash element"?

..
use strict;
use warnings;
...

my (%customer);
%customer = ();
...
 17 sub _customer_id {
 18     my $customer_r = shift;
 19     unless(defined $customer{$customer_r}){
 20         $customer{$customer_r} = ++$customer_id;
 21     }
 22     $customer{$customer_r};
 23 }

I'm just checking whether some hash key exists.

But getting the Use of uninitialized value in hash element at line 19,20,22.

Why?