At first, you could notice there are $26$ ways one can substitute _ with a letter. Trying all combinations would not be solvable in the constraints given in this task. However there are only 3 classes of substitution:

• substitute with a vowel
• substitute with the letter L
• substitute with a consonant different from L

The first class contains $5$ letters, the second class $1$ letter and the third class $20$ letters.

The constraints were small enough that you can now solve the problem by checking all possible combinations of these three classes for each _. Of course you need to take care not to count illegal combinations. A good way to do that is to monitor the last two characters and whether or not L has appeared.