I couldn't tell if your input and output hashes are in form of references or not.

The following is a variant for references:

use strict;
use warnings;
use Data::Dump qw/pp dd/;


my $h1 = {
    'Tim' => {
        'abc' => 7,
        'def' => 7,
        'ghi' => 0
    },
    'Adam' => {
        'abc' => 0,
        'def' => 0,
        'ghi' => 0
    },
    'keas' => {
        'abc' => 0,
        'def' => 0,
        'ghi' => 0
    }
};

my $h2 = {
    'Dav' => {
        'jkl' => 12,
        'mno' => 34,
        'pqr' => 45
    },
    'Adam' => {
        'jkl' => 7,
        'mno' => 0,
        'pqr' => 7
    }
};

my %joined_1st = (%$h1,%$h2);

#pp \%joined_1st;

my $joined_2nd =
    {
        map { $_ => { %{ $h1->{$_} // {} }, %{ $h2->{$_} // {} } } }
            keys %joined_1st
    };


pp $joined_2nd;
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