I am going to suggest a completely different way of doing this.
You know what the script is used for, don't you? Well define your task into 2 interacting jobs:
What you want to do is look at the script and identify something simple which might belong in a more general library that you can easily drop into the script. Found it? Good. Now start a simple module. You can use the following template:
Note that the name of the module should match the package name, in this case My/Module.pm, and where it goes depends on your current development setup.
Now go to the hideous, ugly script and have it use your module. Next take that simple function or bit of functionality which you identified, and put it in your module. Put the name of it in @EXPORT_OK, describe it and then put it in the import list in your script. In other words put a bit in all the things which are, "Will fill in" above. Then replace a bit of the script with your brand new function.
Wash, rinse, and repeat, developing tests, refactoring your library, etc as necessary. At no step are you writing the script from scratch. But you are eating away at it, and eventually there will be little enough left that you can do the rewrite easily. Plus when you get done you have this convenient library.
Note that this approach is not doable with all kinds of scripts. But when it is appropriate, it can be a good path to know about from ugly legacy code to a cleaner design.
In reply to Re (tilly) 1: Best way to fix a broken but functional program?