From the docs:

Declaring an MLDBM hash:

use MLDBM;
use Fcntl;
$dbm = tie %o, 'MLDBM', 'testmldbm', O_CREAT|O_RDWR, 0640 or die $!;
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and modifying an item:

$tmp = $mldb{key};            # retrieve value
$tmp->{subkey}[3] = 'stuff';
$mldb{key} = $tmp;
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which in your case would be something like:

use MLDBM;
use Fcntl;
$hash = tie %o, 'MLDBM', 'testmldbm', O_CREAT|O_RDWR, 0640 or die $!;
...
$tmp = $hash{$nameplace};
if ($male) {
   $tmp->{$person}[0]++;
} else {
   $tmp->{$person}[1]++;
}
$hash{$nameplace} = $tmp;
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(note this is untested...)

rdfield