use Data::Dumper;
my $string = "one 1 two 2 three 3 odd";
my %hash = $string =~ /
                        (\w+)       # one captured word
                        (?:         # followed by
                          \s(\d+)     # a space and a captured number
                        )?          # in an optional non capturing group
                      /gx;
print Dumper \%hash;

__DATA__
$VAR1 = {
          'three' => '3',
          'two' => '2',
          'one' => '1',
          'odd' => undef
        };