Using an integer as a string usually makes for a considerably larger (in memory) scalar than using a string as a number.

How does this fit in with the size that Devel::Size reports? If you can trust it, a stringified number, and a numified string result in the same size (32 byte for the value 123, on a 32-bit Perl).

use Devel::Size qw(size);
use Devel::Peek;

sub info {
    print Dump($_[0]);
    print "size = ",size($_[0]),"  ($_[1])\n\n";
}

$num = 123;  # or int("123")
info($num, "integer");

$num .= "";
info($num, "integer stringified");


$str  = "123";
info($str, "string");

$str += 0;
info($str, "string numified");

$str += 45678900;
info($str, "... with bigger integer");

$str .= "";
info($str, "... re-stringified");
[download]

outputs something like

SV = IV(0x816983c) at 0x8192124
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 123
size = 16  (integer)

SV = PVIV(0x8150b10) at 0x8192124
  REFCNT = 1
  FLAGS = (POK,pPOK)
  IV = 123
  PV = 0x81c9f18 "123"\0
  CUR = 3
  LEN = 4
size = 32  (integer stringified)

SV = PV(0x814fb90) at 0x81ca934
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x81950f0 "123"\0
  CUR = 3
  LEN = 4
size = 28  (string)

SV = PVIV(0x8150b20) at 0x81ca934
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 123
  PV = 0x81950f0 "123"\0
  CUR = 3
  LEN = 4
size = 32  (string numified)

SV = PVIV(0x8150b20) at 0x81ca934
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 45679023
  PV = 0x81950f0 "123"\0
  CUR = 3
  LEN = 4
size = 32  (... with bigger integer)

SV = PVIV(0x8150b20) at 0x81ca934
  REFCNT = 1
  FLAGS = (POK,pPOK)
  IV = 45679023
  PV = 0x81950f0 "45679023"\0
  CUR = 8
  LEN = 12
size = 40  (... re-stringified)
[download]

Devel::Peek shows a comparable resulting structure for "number stringified" and "string numified" (with respect to IV and PV usage). Also, one can observe that the overall size gets larger if you make the number bigger, and then re-stringify the variable...

Anyhow, does your comment mean that Devel::Size is not reporting the size related to the entire PV buffer allocated for the cached stringified form, but rather its currently used part only (up to and including the \0)? — which would make it a less useful tool for determining real memory usage. Actually, the size that Devel::Size reports seems to be related to the LEN in the Devel::Peek dump (which itself you can observe to increment in steps of 4, if you play around a bit). Just wondering...

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