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use strict;
use Benchmark qw/timethese/;
...
        $nth .= $_ if $h{space} == $index-1 && /\S/ .. !/\S/
    }
}

Benchmark: timing 100000 iterations of FindNth_Grantm, FindNth_Jasper,
FindNth_Pg, FindNth_Roger, FindNth_Ysth, FindNth_Zaxo_Array,
...
@ 5396.36/s
FindNth_Zaxo_Split: 14 wallclock secs (14.51 usr +  0.02 sys = 14.53 C
+PU)
@ 6882.31/s

# Pg's approach
my ($nth) = ($str =~ m/(\w+\s*){$index}/);
...
# Grantm's approach
my $idx = $index - 1;
my ($nth) = $str =~ /(?:\w+\W+){$idx}(\w+)/;