The strength of an acid depends upon the oxidation state of the central atom. More is the oxidation state higher is the acidic character.
In case of HClO4(perchloric acid) the oxidation state of chlorine is +7.
And in case of H2SO4 (sulphuric acid) the oxidation state of Sulphur is +6 .
The oxidation state of chlorine in perchloric acid is higher than Sulphur in sulphuric acid so perchloric acid is stronger than sulphuric acid.