Answer
1. The number of oxygen atoms around the central atom:
Perchloric acid $HClO_4$ has 4 oxygens around Cl, and Hypoiodous $HIO$ has only 1.
For oxoacids with same central element: acid strength increases with number of O atoms.
2. The electronegativity of the central atom:
$Cl$ is more electronegative than $I$, and acidity of oxoacids increase with electronegativity of the central atom.
