For speed, I'd suggest:

  sub test6 {
    my $n = $_[0] & $_[1];
    my $count = 0;
    ++$count, $n &= $n - 1 while $n;
    $count;
  }
[download]

For beauty I'd suggest hiding that behind a suitable function name - it costs an extra function call, but you can probably lose that in the way you call it when not benchmarking:

  sub countbits {
    my($n, $count) = ($_[0], 0);
    ++$count, $n &= $n - 1 while $n;
    return $count;
  }
  sub test7 {
    my($iDays, $iCmp) = @_;
    return countbits($iDays & $iCmp);
  }
[download]

You might even want to add some comments. :)

Benchmarks:

          Rate Test4 Test5 Test7 Test6
Test4 106192/s    --  -38%  -40%  -57%
Test5 172463/s   62%    --   -3%  -31%
Test7 176987/s   67%    3%    --  -29%
Test6 248686/s  134%   44%   41%    --
[download]

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