They express the same meanning .

they are the same meaning ,and also you can make other sentence like this“我忘了做作业”和“我忘做作业了”，“我忘了买水果”和“我忘买水果了”

我忘了吃饭“ = i forgot to have a meal (you state you _forgot_ to do it, not someone locked you in the room without giving a snack) 和 ”我忘吃饭了“ = i forgot to have a meal (you state there was such action in that past that you forgot to have a meal, past simple)

一样的意思，只是表达的方式不一样。 就像中国打败日本和中国打胜日本的意思是一样的。

我忘了吃饭 我忘吃饭了 These two sentences share the same meaning,but if you to ask me,I would like to say that they put a slight different emphasis to the sentences..