How to test for "0"

[Zachary Shay]

Is there a way to test for values where zero is valid?

For instance:

%a_Hash;
$a_hash{"user_id"} = 0;
$a_hash{"user_name"} = "root" if ($a_hash{"user_id"});

print $a_hash{"user_id"} if ($a_hash{"user_id"});
print $a_hash{"user_name"} if ($a_hash{"user_name"});

Sometimes the user_id can be undef.  As a result, if there is no user_id...I
don't concern myself with trying to assign a user_name.  The problem is that
I believe the zero is being interpreted as false.  Is there a way to test
these values so that only undef will return as false?

Thanks,
Zach