Se + 6 Cl2 + 4 H2O → H2SeO4 + 6 HCl2
This is an oxidation-reduction (redox) reaction:
Se0 - 6 e- → SeVI (oxidation)
6 HI - 6 e- → 6 HII (oxidation)
12 Cl0 + 12 e- → 12 Cl-I (reduction)
Se is a reducing agent, H2O is a reducing agent, Cl2 is an oxidizing agent.