Before we try to read from a file or try to write to a file
we might want to check our rights, if we can do the required action at all.

For this there is a bunch of so called -X operators. Usually you use them in
an if statement:
if (-e "file.txt") {
    print "File exists !\n";
}
Hence -s can be used either in an if statement or like this:
$size = -s $filename;
There are more such operators see perldoc -f -x