It looks like your question isn't so much about eval than about magical string-to-number conversion.

With regards to option 3 one difference seems to be that $$var will only be replaced by it's value when the eval is invoked. Is there any other behavioural difference when using option 3 instead of option 1?

No, there is no difference. It's all a matter of what eval gets to evaluate.

$var = '09';
$trans = '/3+5';

eval "$var$trans";  # eval gets '09/3+5'
                    # Illegal octal digit '9'

eval $var.$trans;   # eval gets '09/3+5'
                    # Illegal octal digit '9'

eval '$var'.$trans; # eval gets '$var/3+5'
                    # result: 8
[download]

What happens in the last case? Well, eval evaluates $var in a numerical operation (division), in which case the number slot of the variable is used. Its string value still is 09, but its numerical value is 9.

use Devel::Peek;
$var = '09';
Dump $var;
$var + 0;
Dump $var;
$var / 3;
Dump $var;
__END__
SV = PV(0x2e352a0) at 0x2e24fc0
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x2e503f0 "09"\0
  CUR = 2
  LEN = 10
  COW_REFCNT = 0
SV = PVIV(0x254b610) at 0x2e24fc0
  REFCNT = 1
  FLAGS = (IOK,POK,IsCOW,pIOK,pPOK)
  IV = 9
  PV = 0x2e503f0 "09"\0
  CUR = 2
  LEN = 10
  COW_REFCNT = 0
SV = PVNV(0x2ddfc90) at 0x2e24fc0
  REFCNT = 1
  FLAGS = (IOK,NOK,POK,IsCOW,pIOK,pNOK,pPOK)
  IV = 9
  NV = 9
  PV = 0x2e503f0 "09"\0
  CUR = 2
  LEN = 10
  COW_REFCNT = 0
[download]

As you can see, by adding 0 to $var, the integer slot (IV) of the variable is populated with 9. Dividing $var fills the number slot (NV), so the variable is fit for division. The string value (PV) is still 09.