in reply to
How can I re-program the exists function?
Now, my real question is: How can you create a function that would act like the "exists" function?
I'd be happy to be proven wrong, but I'd say you can't. At least not in pure Perl. Maybe with XS code when you hook into the parser.
In some cases you can tie variables, in which exists will call your EXISTS method. But depending on what you actually want to achieve, that might not be a good solution.
So, in the spirit of XY Question, what is the larger problem your are trying to solve?