perl -MData::Dumper -e "while($k=shift @ARGV and $v = shift @ARGV ){$h{$k}=$v};if($k and !$v){$h{$k}=undef}print Dumper \%h " 1 a 2 b ODD

$VAR1 = {
          '1' => 'a',
          'ODD' => undef,
          '2' => 'b'
        };