Thus far in this hour, I've been sort of handwaving over the topic of directory structure. Full pathnames are sometimes needed to open files, and the readdir function can read directories. But actually navigating directories, adding or removing them, and cleaning them out takes a little bit more Perl.
When you run software, your operating system keeps track of what directory you're in when you run the software. When you log in to a Unix machine and run a software package, you are usually placed in your home directory. If you type the operating system command pwd, the shell shows you what directory you are in. If you use MS-DOS or Windows and open a command prompt, the prompt reflects what directory you are in ...