Not so fast. All you've done is compare the results when applying it on a particular string. If you vary the string a bit (removing whitespace, 50 words instead of 100, replacing the 'word' chars with 'non-word' chars), you get different results. Certainly, those strings don't match, but it's important to take the 'time to failure' into account as well.

Below some reworked code. It's still using the same approaches, with the exception of 'Pg's approach. It scored well on the matching case, but if you take out the whitespace in the string, it's too slow to wait for an answer.

use strict;
use Benchmark qw/cmpthese/;
my @str;
my $index = 82;

# construct a small string
foreach (1 .. 100) {
    $str [0] .= "Element$_" . ' ';
    $str [1] .= "Element$_";
}
foreach (1 .. 50) {
    $str [2] .= "Element$_" . ' ';   
    $str [3] .= "Element$_";
}
 
$str [4] = "*&()-=-_<> " x 100;
$str [5] = "*&()-=-_<>"  x 100;
$str [6] = "*&()-=-_<> " x  50;
$str [7] = "*&()-=-_<>"  x  50;

my @info = (
    "100 words separated by whitespace",  
    "Same, without the whitespace",
    "50 words separated by whitespace",
    "Same, without the whitespace",
    "100 non-words separated by whitespace",
    "Same, without the whitespace",
    "50 non-words separated by whitespace",
    "Same, without the whitespace",
);
 
my $str;

for (my $i = 0; $i < @str && $i < @info; $i ++) {
    print $info [$i], "\n";
    $str = $str [$i];

    cmpthese (-1, {
      Roger      => \&FindNth_Roger,
      Zaxo_Array => \&FindNth_Zaxo_Array,
      Zaxo_Split => \&FindNth_Zaxo_Split,
      Ysth       => \&FindNth_Ysth,
#     Pg         => \&FindNth_Pg,
      Grantm     => \&FindNth_Grantm,
      Jasper     => \&FindNth_Jasper,
    });
}

sub FindNth_Roger() {
    my $nth = @{[$str =~ m/\w+/g]}[$index-1];
}

sub FindNth_Zaxo_Array() {
    my $nth = ($str =~ m/\w+/g)[$index-1];
}
    
sub FindNth_Zaxo_Split() {
    my $nth = (split ' ', $str)[$index-1];  
}
    
sub FindNth_Ysth() {
    my $nth = [$str =~ m/\w+/g]->[$index-1];
}

sub FindNth_Pg() {
    my ($nth) = ($str =~ m/(\w+\s*){$index}/);   
}
    
sub FindNth_Grantm() {
    my $idx = $index - 1;
    my ($nth) = $str =~ /(?:\w+\W+){$idx}(\w+)/;
}
      
sub FindNth_Jasper() {
    my $nth;
    my %h;
    for ($str=~/.?/g) {
        $h{space} += !/\S/;
        $nth .= $_ if $h{space} == $index-1 && /\S/ .. !/\S/
    }
}
    
__END__
100 words separated by whitespace
            (warning: too few iterations for a reliable count)
              Rate    Jasper     Roger      Ysth Zaxo_Array Zaxo_Split
+    Grantm
Jasper       429/s        --      -95%      -95%       -96%       -98%
+      -98%
Roger       8614/s     1910%        --       -0%       -19%       -54%
+      -69%
Ysth        8650/s     1918%        0%        --       -19%       -54%
+      -69%
Zaxo_Array 10676/s     2391%       24%       23%         --       -43%
+      -62%
Zaxo_Split 18797/s     4286%      118%      117%        76%         --
+      -33%
Grantm     27926/s     6416%      224%      223%       162%        49%
+        --
Same, without the whitespace
               Rate    Jasper   Grantm     Roger      Ysth Zaxo_Array 
+Zaxo_Split
Jasper        491/s        --     -93%     -100%     -100%      -100% 
+     -100%
Grantm       6636/s     1250%       --      -94%      -94%       -95% 
+      -95%
Roger      110277/s    22340%    1562%        --       -4%       -22% 
+      -23%
Ysth       114840/s    23269%    1631%        4%        --       -19% 
+      -20%
Zaxo_Array 141940/s    28783%    2039%       29%       24%         -- 
+       -1%
Zaxo_Split 143360/s    29072%    2060%       30%       25%         1% 
+        --
50 words separated by whitespace
              Rate    Grantm    Jasper     Roger      Ysth Zaxo_Array 
+Zaxo_Split
Grantm       610/s        --      -32%      -96%      -96%       -97% 
+      -98%
Jasper       896/s       47%        --      -95%      -95%       -96% 
+      -97%
Roger      16440/s     2595%     1734%        --       -1%       -20% 
+      -54%
Ysth       16593/s     2620%     1751%        1%        --       -19% 
+      -53%
Zaxo_Array 20479/s     3257%     2185%       25%       23%         -- 
+      -42%
Zaxo_Split 35544/s     5727%     3866%      116%      114%        74% 
+        --
Same, without the whitespace
               Rate    Jasper   Grantm     Roger      Ysth Zaxo_Array 
+Zaxo_Split
Jasper        984/s        --     -93%      -99%      -99%      -100% 
+     -100%
Grantm      13398/s     1262%       --      -92%      -92%       -94% 
+      -94%
Roger      165414/s    16714%    1135%        --       -6%       -29% 
+      -31%
Ysth       175812/s    17770%    1212%        6%        --       -25% 
+      -26%
Zaxo_Array 234057/s    23691%    1647%       41%       33%         -- 
+       -2%
Zaxo_Split 238602/s    24153%    1681%       44%       36%         2% 
+        --
100 non-words separated by whitespace
              Rate    Jasper     Roger      Ysth Zaxo_Array Zaxo_Split
+    Grantm
Jasper       393/s        --      -96%      -96%       -97%       -98%
+      -99%
Roger       9050/s     2206%        --        0%       -20%       -51%
+      -67%
Ysth        9050/s     2206%        0%        --       -20%       -51%
+      -67%
Zaxo_Array 11270/s     2771%       25%       25%         --       -39%
+      -60%
Zaxo_Split 18442/s     4598%      104%      104%        64%         --
+      -34%
Grantm     27836/s     6992%      208%      208%       147%        51%
+        --
Same, without the whitespace
               Rate    Jasper      Ysth     Roger Zaxo_Array   Grantm 
+Zaxo_Split
Jasper        432/s        --      -95%      -95%       -96%     -98% 
+     -100%
Ysth         9050/s     1993%        --       -0%       -20%     -67% 
+      -93%
Roger        9050/s     1993%        0%        --       -20%     -67% 
+      -93%
Zaxo_Array  11270/s     2506%       25%       25%         --     -60% 
+      -91%
Grantm      27836/s     6337%      208%      208%       147%       -- 
+      -79%
Zaxo_Split 130327/s    30038%     1340%     1340%      1056%     368% 
+        --
50 non-words separated by whitespace
              Rate    Grantm    Jasper     Roger      Ysth Zaxo_Array 
+Zaxo_Split
Grantm       369/s        --      -53%      -98%      -98%       -98% 
+      -99%
Jasper       785/s      113%        --      -95%      -95%       -96% 
+      -98%
Roger      17231/s     4568%     2095%        --       -0%       -22% 
+      -51%
Ysth       17300/s     4586%     2104%        0%        --       -21% 
+      -51%
Zaxo_Array 21976/s     5853%     2699%       28%       27%         -- 
+      -38%
Zaxo_Split 35484/s     9512%     4420%      106%      105%        61% 
+        --
Same, without the whitespace
               Rate    Grantm   Jasper     Roger      Ysth Zaxo_Array 
+Zaxo_Split
Grantm        410/s        --     -52%      -98%      -98%       -98% 
+     -100%
Jasper        861/s      110%       --      -95%      -95%       -96% 
+     -100%
Roger       17230/s     4101%    1902%        --       -1%       -22% 
+      -92%
Ysth        17397/s     4142%    1922%        1%        --       -21% 
+      -92%
Zaxo_Array  21976/s     5259%    2454%       28%       26%         -- 
+      -90%
Zaxo_Split 219429/s    53407%   25398%     1174%     1161%       899% 
+        --
[download]

Note how 'Grantm' sometimes is the fastest, sometimes is the slowest.

Abigail