This uses a look behind ((?<=")) to match the " and a \b to make sure the " is followed by a "word" character.

use warnings;
use strict;

while (<DATA>) {
  s/(?<=")\b/..\//;
  print;
}

__DATA__
if (-e "test2122.asm")
[download]

Prints:

if (-e "../test2122.asm")
[download]