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     s/iter   org   new  best
org    5.62    --  -78%  -94%
new    1.26  346%    --  -73%
best  0.345 1529%  266%    --
The results are the same

sub ba2bi2 {
    my @ba = reverse (@_, (0) x (4 - ( @_%4 || 4 ) ) );
...
            } 0 .. ($#ba/4);
    return $result;
}