undef
All scalars that have not been explicitly defined have the value of undef. You can also assign this value to a scalar. In this way, undef is pretty much like NULL...
my $a; my $b=undef;
The thing is, you can't explicitly check for undef, so no
if ($b==undef) {...} # no
So, instead you need to
if (defined($b)) {...} # yes
I suppose that would be is_not_null or whatever. This is handy for setting and checking parameters to a function that aren't always needed.
