${\mathrm{MnO}}_2+4\mathrm{HCl} \to {\mathrm{MnCl}}_2+{\mathrm{Cl}}_2+2{\mathrm{H}}_2\mathrm{O}$, In the reaction ${\mathrm{MnO}}_2$ acts as

In the given reaction $\mathrm{HCl}$ is converted to ${\text{Cl}}_2$, which involves removal of hydrogen atoms. In other words the oxidation number of chlorine is changed from $-1$ to $0$, which is an oxidation reaction. ${\mathrm{MnO}}_2$ itself gets reduced and influences the oxidation reaction by converting $\mathrm{HCl}$ to ${\mathrm{Cl}}_2$. So ${\mathrm{MnO}}_2$ acts as an oxidant in the given reaction.