#!/usr/bin/perl

use Data::Dumper;

$hash = {
    abc => {
        'def' => 1,
    },
    ghi => {
        jkl => 1,
        mno => 1,
    },
};
print Dumper(\$hash);
delete($hash->{abc}->{def});
print Dumper(\$hash);
__END__
$VAR1 = \{
            'abc' => {
                       'def' => 1
                     },
            'ghi' => {
                       'mno' => 1,
                       'jkl' => 1
                     }
          };
$VAR1 = \{
            'abc' => {},
            'ghi' => {
                       'mno' => 1,
                       'jkl' => 1
                     }
          };