Multiple problems here.

1. $OldVariable contains characters special to the regexp engine -- prepend with \Q.
2. The random number may be repeated.
3. Once you've modified $code, @code isn't updated.
4. Your code doesn't deal with multiple arrays on a line.
5. The sigil for (scalar) array access is $, not @.
6. You aren't modifying any arrays that aren't indexed (for instance, your code leaves my @array untouched).
7. You're assuming there's no space between the array name and the index.
8. You're assuming the index consists of just a number, what if it's an expression, a variable, or has whitespace?
9. You're missing all the arrays containing capital letters, or that start with an underscore.