Poke some holes to the bottom, don't go crazy on it but 4 or 5 won't hurt, bamboo stakes work great.

Perlite is something that is mixed into the soil medium upon the start of the grow. When you say you added perlite to it at week 4 of flower what exactly are you referring to? Did you pull the plant out of the pot to add perlite? 4th week of flower that pot should have already started to totally fill with roots. How did you do it? Was it a top dressing? That wouldn't do anything. Plastic buckets will work but they are not always the greatest when it comes to draining and breathing. I use fabric pots. Drill more holes in the bottom of the bucket and poke some holes in the soil with sticks like the other guy was saying. That is your best bet since its pretty much too late to do anything else. I have used MG many times over the years for growing flower and although there is much more preferred soils out there you can still grow beautiful flowers with it so long as its the yellow bag. Gotta be potting soil not garden soil. This being said I don't think its a direct issue with your soil. Perhaps you packed the soil too tight in the bucket this can reduce air space in the pot which slows drainage big time. Thats all I got. Best of luck to you!