Easy to test:

>perl -wMstrict -MData::Dump -le
"use Readonly;
 ;;
 Readonly my %h => (a => {b => {c => {d => 1}}});
 dd \%h;
 ;;
 $h{a}{b}{d} = 2;
"
{
  # tied Readonly::Hash
  a => {
         # tied Readonly::Hash
         b => {
                # tied Readonly::Hash
                c => {
                       # tied Readonly::Hash
                       d => 1,
                     },
              },
       },
}

Modification of a read-only value attempted at -e line 1
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See also DESCRIPTION section of Readonly.