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use strict;
use warnings;
...
   Two   => sub { popcount2($_) for $start .. $end },
   Three => sub { popcount3($_) for $start .. $end },
};

sanity test for correctness
Benchmark: timing 50 iterations of One, Three, Two...
   One: 29 wallclock secs (28.41 usr +  0.00 sys = 28.41 CPU) @  1.76/
+s (n=50)
 Three: 10 wallclock secs (10.00 usr +  0.00 sys = 10.00 CPU) @  5.00/
+s (n=50)
   Two: 10 wallclock secs (10.03 usr +  0.00 sys = 10.03 CPU) @  4.98/
+s (n=50)

// Make GCC __builtin_popcountll available with MSVC
// (used to calculate the Hamming weight efficiently)
...
#define __builtin_popcountll __popcnt64
#define __builtin_popcount   __popcnt
#endif

   # $cnt += popcount($row->[$iy] & $BM_MIDDLE);
   $cnt += sprintf('%b', $row->[$iy] & $BM_MIDDLE) =~ tr/1//;