I certainly learned that lesson when I had to move a program from Unix to Windows ... and to maintain it, for a time, in both environments simultaneously.   I don’t know who wrote the original code, and that is probably a good thing, because I think that I would have a word with him or her...

Within CPAN, there are idioms that allow you to do various operating-system specific things in a non-OS specific way, just by virtue of using a CPAN module that will install itself appropriately in different environments.   Now, your code is portable.   And, if you peer inside the guts of some of these modules, you find that the solutions that they implement are often much more rugged and thorough than what you would have come up with on your own steam.