File test or -X operators

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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";
}

• -e File (or directory) exists
• -r File (or directory) is readable by this user
• -w File (or directory) is writable by this user
• -x File (or directory) is executable by this user
• -f Entry is a file
• -d Entry is a directory
• -l Entry is a symbolic link
• -s Size of the file (hence also means 'file is not empty')
• -M Number of days between the modification date of a file and the start time of our script

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`