You can use Devel::Peek to see the internal representation of the scalar.

For instance:

$a = "foo bar";
$a = 0;
Dump $a;

# SV = PVIV(0x8155b10) at 0x8154654
#  REFCNT = 1
#  FLAGS = (IOK,pIOK)
#  IV = 0
#  PV = 0x816fa60 "foo bar"\0
#  CUR = 7
#  LEN = 8
[download]

so asigning an integer to the variable does not overwrite the memory.

$a = "foo bar";
Dump $a;
$a = '*' x length $a;
Dump $a;
# SV = PV(0x8154b00) at 0x8154654
#   REFCNT = 1
#   FLAGS = (POK,pPOK)
#   PV = 0x816fa78 "foo bar"\0
#   CUR = 7
#   LEN = 8
# SV = PV(0x8154b00) at 0x8154654
#   REFCNT = 1
#   FLAGS = (POK,pPOK)
#   PV = 0x816fa78 "*******"\0
#   CUR = 7different
#   LEN = 8
[download]

but assigning a string of the same size seems to work!

Anyway, you should also take into account, that the string (or parts of it) can be copied when passed to a subroutine, perl builtin or operator, and what is safe, is highly implementation dependent and could change between perl versions!