%hash = {
         abc => {
                 def => 1,
                },
         ghi => {
                 jkl => 1,
                },
};

##</code><code>##

delete($hash{abc}{def});

##</code><code>##

%hash = {
         abc => undef,
         ghi => {
                 jkl => 1,
                 mno => 1,
                },
};

##</code><code>##

%hash = {
         ghi => {
                 jkl => 1,
                 mno => 1,
                },
};