Interesting post ... it prompted me to run the following:

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

$str  = "123";
$str += 0; # numify $str

$num = int("123");

print size($str), " ", size($num), "\n";
[download]

On Win32 that outputs 32 16 (56 24 on a 64-bit build of perl). I guess that's a reliable way of determining the memory consumed by both $str (32 bytes) and $num (16 bytes). Zat right ?

Cheers,
Rob