You want to read the output of perldoc -f -X (the "-f" tells perldoc to display the portion of the massive perlfunc manual that explains all the '-X' built-in functions, which include: -e -d, -f, -s, -r, -M and many other useful things.

The '-X' functions are all concerned with getting details about files: -d $filename returns true if $filename exists and is a directory; -f $filename returns true if $filename exists and is a data file.