Is that a bug or am I missing an obscure reason for this???

 DB<18> use Scalar::Util qw/readonly/

 DB<19> p readonly 1
34283264
 DB<20> p readonly undef
34283264
 DB<21> p readonly {}                     # What?

 DB<22> p readonly []                     # ...

 DB<23> p readonly bless([],"TEST")       # ...

 DB<24> x [],{},bless([],"TEST")          # all refs
  ARRAY(0x334ebd0)
    empty array
  HASH(0x334ec00)
    empty hash
  TEST=ARRAY(0x334eb70)
    empty array
 DB<25> x map { $_ = 1 } [],{},bless([],"TEST") # mutable! WHY???
  1
  1
  1
 DB<26>
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update

counter-examples

  DB<32> x map { $_ = 1 } "str"
Modification of a read-only value attempted at (eval 42)[c:/Perl_524/l
+ib/perl5db.pl:737] line 2.
  DB<33> x map { $_ = 1 } 42
Modification of a read-only value attempted at (eval 43)[c:/Perl_524/l
+ib/perl5db.pl:737] line 2.
  DB<34> x map { $_ = 1 } undef
Modification of a read-only value attempted at (eval 44)[c:/Perl_524/l
+ib/perl5db.pl:737] line 2.
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update

see also Re^10: Shouldn't references be readonly? (UPDATED) for another example of the asymmetric behavior.