Interesting, but it appears you are abusing the expectations of an experienced command-line user. As a matter of fact you're not even processing the integer in question with Getopt.

2 more traditional solutions come to mind:

Use the options terminator:

$ add.pl -- -5
$ add.pl -offset -2 -- 12
[download]

or use the "pass_through" option for configuration instead.

$ add.pl -5
$add.pl -offset -2 -5
[download]

Since you're shifting $value off of the command line anyway this really seems the best way.