The acidic strength of the oxoacids of a given halogen increases with an increase in the oxidation state of the central halogen atom. The oxidation states of chlorine in $HClO$, $HClO_2$, $HClO_3$, and $HClO_4$ are +1, +3, +5, and +7, respectively. As the number of highly electronegative oxygen atoms attached to the chlorine atom increases, the electron density of the O-H bond is pulled more strongly towards the Cl-O part of the molecule, weakening the O-H bond and facilitating the easy release of a proton ($H^+$). Additionally, the stability of the corresponding conjugate bases ($ClO^- < ClO_2^- < ClO_3^- < ClO_4^-$) increases due to the dispersal of the negative charge over more oxygen atoms via resonance. Therefore, the acidity order is $HClO < HClO_2 < HClO_3 < HClO_4$.