So I was able to reproduce it

  DB<75> use Readonly;

  DB<77> Readonly::Hash    %has => (key1 => 1, key2 => 2 );

  DB<78> x \%has
0  HASH(0x332b388)
   'key1' => 1
   'key2' => 2
  DB<79> $a{1}=\%has

  DB<80> use warnings; say $a{1}{foo}{bar}
Modification of a read-only value attempted at (eval 94)[c:/Perl_524/l
+ib/perl5db.pl:737] line 2.

  DB<81>
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you must check if there is any key {creden} before trying to access a deeper key.

no autovivification is another possibility.