while the explanation of Eliya++ pretty much solves the issue, I noticed a strange behaviour. if I do exactly the same but using a tied hash, Perl seems to parse the instruction differently. no key with undef value is created at all.

#!/usr/bin/env perl

use 5.010;

use warnings;
use strict;

use Tie::Hash;
use Data::Dumper;

package DEBUGHASH;

use base 'Tie::Hash';

sub TIEHASH {
    my $self = bless {}, shift;
    warn "TIEHASH $self\n";
    return $self;
}

sub STORE {
    my($self, $key, $val) = @_;
    warn "STORE $self $key=$val\n";
    $self->{$key} = $val;
}

sub FETCH {
    my($self, $key) = @_;
    warn "FETCH $self $key\n";
    return $self->{$key};
}

sub EXISTS {
    my($self, $key) = @_;
    warn "EXISTS $self $key\n";
    return exists $self->{$key};
}

sub FIRSTKEY {
    my($self) = @_;
    warn "FIRSTKEY $self\n";
    my $key = scalar keys %{$self}; 
    return each %{$self};
}

sub NEXTKEY {
    my($self, $lastkey) = @_;
    warn "NEXTKEY $self $lastkey\n";
    return each %{$self};
}

package main;

my %x;
tie %x, 'DEBUGHASH';

$x{a} &&= 1;

# this one, instead, generates the 'a' => undef key
# $x{a} = $x{a} && 1;

say Dumper(\%x);

# just to check that the hash really works...
$x{b} = 42;

say Dumper(\%x);
[download]

running the code above I get:

TIEHASH DEBUGHASH=HASH(0x649f78)
FETCH DEBUGHASH=HASH(0x649f78) a
FIRSTKEY DEBUGHASH=HASH(0x649f78)
$VAR1 = {};

STORE DEBUGHASH=HASH(0x649f78) b=42
FIRSTKEY DEBUGHASH=HASH(0x649f78)
FETCH DEBUGHASH=HASH(0x649f78) b
NEXTKEY DEBUGHASH=HASH(0x649f78) b
$VAR1 = {
          'b' => 42
        };
[download]

probably not worth further investigation, but curious nonetheless :-)