HClVIIO4 + 4 SIVO2 + 4 H2O 4 H2SVIO4 + HCl-I

This is an oxidation-reduction (redox) reaction:

4 SIV - 8 e- 4 SVI (oxidation)

ClVII + 8 e- Cl-I (reduction)

SO2 is a reducing agent, HClO4 is an oxidizing agent.