use feature qw( say );

use Devel::Size qw( total_size );
use Devel::Peek qw( Dump );

my $num = 1000;
say total_size($num);
Dump($num);

say '';

my $packed = pack("w*",$num);
say total_size($packed);
Dump($packed);
[download]

---

5.12, 64 bit

24
SV = IV(0x768010) at 0x768018    <-- See SVt_IV image below
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 1000                      <-- 8 bytes

48
SV = PV(0x752f28) at 0x768000    <-- See SVt_PV image below
  REFCNT = 1                            Three more fields than SVt_IV
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x7d6520 "\207h"\0
  CUR = 2                        <-- Even though only 3 are needed,
  LEN = 8                        <-- 8 byte string buffer allocated
[download]

SVt_IV, SVt_PV

Are the greyed out fields outside of allocated memory or simply unused? [ See dave_the_m's reply ]

(The numbers still don't quite add up, but it should give you an idea.)

---

5.14, 32 bit

16
SV = IV(0x51a100) at 0x51a104    <-- See SVt_IV image below
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 1000                      <-- 4 bytes

36
SV = PV(0x5e8cc4) at 0x51a054    <-- See SVt_PV image below
  REFCNT = 1                            Four more fields than SVt_IV
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x5b4334 "\207h"\0
  CUR = 2                        <-- Even though only 3 are needed,
  LEN = 12                       <-- 12 byte string buffer allocated
[download]

SVt_IV, SVt_PV

(The numbers still don't quite add up, but it should give you an idea.)

Update: Adjusted based on dave_the_m's explanation.