my $lt = [];
$lt->[ord(a)] = 00;
$lt->[ord(b)] = 01;
$lt->[ord(c)] = 10;
$lt->[ord(d)] = 11;

foreach my $string{
_convert($string);

...
sub _convert{
my $string = shift;
my $bs="";
foreach(split(//,$string)){

$bs.=$lt->[$_];
}

return int($bs);
}