my %hash;
my $a = undef;
my $b = '';
$hash{$a} = 'foo';
print "$hash{$a}\n";
print "$hash{$b}\n";
<code>

<p>Of course both these throw a warning.
<code>
C:\temp>hash.pl
Use of uninitialized value in hash element at C:\temp\hash.pl line 8.
Use of uninitialized value in hash element at C:\temp\hash.pl line 9.
foo
foo