for ([split /(\s+)/, $str]) {
    shift @$_ if $_->[0] eq '';
    my ($s,$e)= do {
        my @c=@$_;
        map $_ + ($c[$_] =~ /\s/ ?
            ($_ >=0 ? 1 : -1 ) : 0) => 0, -1;
    };
    $_->[$s] ^= $_->[$e];
    $_->[$e] ^= $_->[$s];
    $_->[$s] ^= $_->[$e];
    y/\0//d for @$_;
    local $"='';
    print "@$_";
}