Can I say in the poem: "I will keep your heart from the fall"? Or "I will save your heart from the fall'? Of course, it's indirect meaning.
If you are referring to a particular fall, then yes, you can say:
I will keep your heart from the fall.
But if you don't have a particular fall in mind, and the meaning you want to convey is that you will take care so that the person's heart never falls, you can say:
I will keep your heart from falling.
P.S. So you want to ask, "Can I say in a poem..." or "... in my poem", because we, the readers|listeners, don't know which poem you are referring to when you say "the poem". After you have made clear to us which poem you have in mind (e.g "my poem"), then you can begin to say "the poem".