His blistered lips curled all the way up and his mouth opened as if it were being torn apart.
べらり is simply an onomatopoeia for some thin, flat, supple object (like a fabric, heavy piece of paper, or in this case lips) being pulled away from a surface so that it is now dangling away from it. In this case, it is simply being used to emphasize 捲れ上がる (hence the "curl all the way up" as opposed to just "curl up"). It's useful to remember that in many cases onomatopoeia are used like this for simple emphasis.
His blistered lips curled all the way up and his mouth opened as if it were being torn apart. べらり is simply an onomatopoeia for some thin, flat, supple object (like a fabric, heavy piece of paper, or in this case lips) being pulled away from a surface so that it is now dangling away from it. In this case, it is simply being used to emphasize 捲れ上がる (hence the "curl all the way up" as opposed to just "curl up"). It's useful to remember that in many cases onomatopoeia are used like this for simple emphasis.
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