How many years ago could I have written a program in this language and still have it work today? If I don't use excessive libraries, how many distributions wil

How many years ago could I have written a program in this language and still have it work today? If I don't use excessive libraries, how many distributions will my programs run on reliably without a tambourine dance? If I write my code in a certain restricted style, can I still understand it after a year of not looking at it? How likely is my code to work, at least partially after it is put through various bit-rot processes? How much existing code example and solutions is there on the web, outside of StackOverflow, which still works when used today? (I chose Perl for my project.)