Said mechanism is to make the SV readonly. 
>perl -MDevel::Peek -e"Dump('Hello Sailor')"
SV = PV(0x228e68) at 0x1831670
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,POK,READONLY,pPOK)
                              ^^^^^^^^
  PV = 0x1833f3c "Hello Sailor"\0
  CUR = 12
  LEN = 16

[download]

Obviously that mechanism won't help you here.

In fact, a flag alone won't work since you'd need to know how many times a variable has been aliased to know when to remove the flag. (Upd: I'm not sure about the preceding statement. Can't think straight right now. )

You could add magic to the variable like taint does. 

>perl -T -MDevel::Peek -e"Dump($ARGV[0])" foo
SV = PVMG(0x1822dcc) at 0x226ba0
  REFCNT = 1
  FLAGS = (GMG,SMG,pPOK)
  IV = 0
  NV = 0
  PV = 0x182d324 "foo"\0
  CUR = 3
  LEN = 4
  MAGIC = 0x182d33c
    MG_VIRTUAL = &PL_vtbl_taint
    MG_TYPE = PERL_MAGIC_taint(t)
    MG_LEN = 1

[download]

But now you're getting into costly.

In reply to Re^8: Changing array by changing $_? by ikegami
in thread Changing array by changing $_? by johnvandam

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