Also, I've assumed "/" is a correct filepath separator and in most cases it is, but if you want to write really portable code you'll want to look into File::Spec.

That's not the complete picture, as non-Unix operating systems support POSIX syntax (i.e. Unix) besides their own native syntax. All operating systems that run perl support POSIX syntax, otherwise the perl build process wouldn't work. For a bizarre native syntax, have a look at VMS. There can be problems if you try and mix native and POSIX, though sometimes in Windows, you can get away with a mixture of slashes leaning both ways.

My advice is to either stick to native syntax, with File::Spec and friends, or use POSIX syntax throughout. Beware that a user supplied filename might contain native syntax elements.

For more on this, and portability in general, please refer to my talk slides - I have given this talk at several YAPCs.