my %hoh = (
    a => {
        a1 => {
            a11 => 1,
            a12 => 1,
        },
        b1 => {
            b11 => 1,
            b12 => 1, 
        },
        c1 => 1,  
    },
    b => {
        --- some elements like above---
    },
);