Yes, the current working directory can change, but that's the best guess we have. The only thing better might be to get the script's cwd at the beginning, so we'd insert it in the BEGIN { } section, I guess.

In Windows, what happens often is you execute a script and Windows will supply perl.exe with the full path of the script. So, that's why in most cases in Windows $0 will yield a full absolute path but not always! If a perl script happens to be in the same directory where perl.exe resides and you were to open command prompt and enter "perl.exe myscript.pl" then $0 would only give you the name of the script. And in this case, if the script looks for the current working directory, then it would find itself.