aleph:~ [19:27:57]$ perl -wMData::Dumper -e 'print Dumper {undef() => undef}'
Use of uninitialized value in anonymous hash ({}) at -e line 1.
$VAR1 = {
          '' => undef
        };

##</code><code>##

my %h = map { defined ? $_ : $default }
        map { chomp; (split /\s+/, $_, 2)[0,1] } <DATA>;


##</code><code>##

my %pairs = map { chomp; 
    my($k,$v)=(split ' ', $_, 2)[0,1];
    defined $k ? ($k => $v // $default) : () } <DATA>;

##</code><code>##

my %pairs = map { chomp; 
    map @{ ([], [@$_, $default], $_)[scalar @$_] }, 
    [split ' ', $_, 2] } <DATA>;