It might be worth pointing out that even the module Readonly will - contrary to what one might expect - not prevent the internal structures from being modified by implicit type conversions. (In addition, the structures are more complex to start with.)  E.g.

use Devel::Peek;
use Readonly;

Readonly::Scalar  my $number => 10;

Dump($number);
printf "Number is %d\n", $number;
Dump($number);
print "Number is $number\n";
Dump($number);
[download]

Output:

SV = PVMG(0x68ba80) at 0x65eb40
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,GMG,SMG,RMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x662c90
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedscalar(q)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x63c410
    SV = RV(0x6ba0d0) at 0x63c410
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x6dc2e0
      SV = PVMG(0x68ba48) at 0x6dc2e0
        REFCNT = 1
        FLAGS = (PADBUSY,PADMY,OBJECT,IOK,pIOK)
        IV = 10
        NV = 0
        PV = 0
        STASH = 0x6dc120	"Readonly::Scalar"
Number is 10
SV = PVMG(0x68ba80) at 0x65eb40
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,GMG,SMG,RMG,pIOK)
  IV = 10
  NV = 0
  PV = 0
  MAGIC = 0x662c90
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedscalar(q)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x63c410
    SV = RV(0x6ba0d0) at 0x63c410
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x6dc2e0
      SV = PVMG(0x68ba48) at 0x6dc2e0
        REFCNT = 1
        FLAGS = (PADBUSY,PADMY,OBJECT,IOK,pIOK)
        IV = 10
        NV = 0
        PV = 0
        STASH = 0x6dc120	"Readonly::Scalar"
Number is 10
SV = PVMG(0x68ba80) at 0x65eb40
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,GMG,SMG,RMG,pIOK,pPOK)
  IV = 10
  NV = 0
  PV = 0x6519b0 "10"\0
  CUR = 2
  LEN = 8
  MAGIC = 0x662c90
    MG_VIRTUAL = &PL_vtbl_packelem
    MG_TYPE = PERL_MAGIC_tiedscalar(q)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0x63c410
    SV = RV(0x6ba0d0) at 0x63c410
      REFCNT = 1
      FLAGS = (ROK)
      RV = 0x6dc2e0
      SV = PVMG(0x68ba48) at 0x6dc2e0
        REFCNT = 1
        FLAGS = (PADBUSY,PADMY,OBJECT,IOK,pIOK)
        IV = 10
        NV = 0
        PV = 0
        STASH = 0x6dc120	"Readonly::Scalar"

_____

Update: ...similarly with Scalar::Readonly, btw  (though here the internal structures are as lean as without using the module):

use Devel::Peek;
use Scalar::Readonly ':all';

my $number = 10;
readonly_on($number);

Dump($number);
print "Number is $number\n";
Dump($number);
[download]

SV = IV(0x661dc8) at 0x65eb50
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,READONLY,pIOK)
  IV = 10
Number is 10
SV = PVIV(0x63e130) at 0x65eb50
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,POK,READONLY,pIOK,pPOK)
  IV = 10
  PV = 0x654fb0 "10"\0
  CUR = 2
  LEN = 8