I think it depends on how you initialize %booklist_1.

In the code below, Init_1 seems to reproduce your problem, while Init_2 provides an alternative, properly functioning initialization.

I remember running into this earlier, and posting a very similar issue in Conquering "x" operator over-optimization. merlyn gently indicated that a "deep copy" is required in this case.

use strict;
use warnings;

my ($book_id , %booklist_1);

Init_1();
populate_and_print();

Init_2();
populate_and_print();
#--------------------

sub populate_and_print{
    foreach $book_id (keys %booklist_1) {
    
        my ($update_id, $title) = ( int (100 *rand()), "title" . int (
+1000 *rand()));
        $booklist_1{$book_id}{'update_id'} = $update_id;
        $booklist_1{$book_id}{'title'} = $title;
    
        print "$book_id > $booklist_1{$book_id}{'update_id'}\n";
    }
    #$sth->finish;
    
    foreach $book_id (keys %booklist_1) {
        print "XXX $book_id > $booklist_1{$book_id}{'update_id'}\n";
    }
}

sub Init_1{
  %booklist_1=();
  @booklist_1{ 100..104 }
    = ( { title => 'empty', update_id => 'missing' } )
      x 5;
}

sub Init_2{
  %booklist_1=();
  $booklist_1{$_ }
    =  { title => 'empty', update_id => 'missing' } 
      for 100..104;
}
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