Try this alternative code:

use strict;
use warnings;
use Data::Dumper;


my $hash;

while (<DATA>){
    my ($left,$val) = split /=/,$_ or next;
    my ($first,@levels) = split /\./,$left;
    
    my $str="\$hash->{$first}";
    
    for my $last_level(@levels){
        $str .= "{$last_level}" 
    }
    eval "$str=$val";
}

print Dumper \$hash;

__DATA__
a.b='xyz'
a.c='abc'
d='def'
a.e='ghi'
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If using for production, you may want to sanitize the data before the string eval.
Output:

$VAR1 = \{
            'a' => {
                     'e' => 'ghi',
                     'c' => 'abc',
                     'b' => 'xyz'
                   },
            'd' => 'def'
          };
[download]