$ perl -E 'use Data::Dumper; my %x; $x{a}&&=1; say Dumper(\%x);'

$VAR1 = {
          'a' => undef
        };


##</code><code>##

This is perl 5, version 12, subversion 4 (v5.12.4) built for x86_64-linux-gnu-thread-multi