srand();
my @salts = ( "A".."Z", "a".."z", "0".."9", ".", "/" );
my $salt = $salts[int(rand(64))] . $salts[int(rand(64))];

$crypt = crypt($str, $salt);

こういうルーチンだと、暗号化文字列内に「_」は絶対含まれ
ないですよね？