#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %hash;
my $str = "one 1 two 2 three 3 odd_element";
my @array = split / /, $str;

while (@array) {
    my $key = shift @array;
    my $value = shift @array;
    if (defined $key && defined $value) {
	$hash{$key} = $value;
    }
    else {
	$hash{$key} = undef;
    }
}

print Dumper \%hash;