my @lookup;

for( 0 .. 65535 ) {
  my $bits = sprintf "%b", $_;
  my $count = $bits =~ tr/1/1/;
  push @lookup, $count;
}

my $bitstring = '';

vec( $bitstring, 0, 32 ) = 1234567891;

my $count = 0;

for( 0 .. length( $bitstring ) / 2 - 1 ) {
  $count += $lookup[ vec( $bitstring, $_, 16 ) ];
}

print $count, "\n";