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

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