The key must be undef.

lanx@nc10-ubuntu:~$ perl
use strict;
{ use warnings;
  sub test { my $x = shift; }
}
my %hash = ( a => "a" );
my $key;
test $hash{$key}
Use of uninitialized value in scalar assignment at - line 3.

lanx@nc10-ubuntu:~$ perl -version

This is perl, v5.10.0 built for i486-linux-gnu-thread-multi
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I agree with salva that this is kind of a bug in lexical warnings.