What ions and/or molecules (apart from water) are present in relatively large proportions in a solution of a weak acid hclo (aq) ?

HClO, perchloric acid is a weak acid. Unlike strong acids like HCl or H2SO4 it not dissociate completely but partially such that an equilibrium exists between the dissociated ions and the undissociated acid. The equilibrium is as shown below:

HClO + H2O ↔ H3O⁺ + ClO⁻

Since HClO is a weak acid, the reverse reaction is favored over the forward reaction. Thus apart from water, HClO will be present in large amounts.