Item 104. Track dangerous data with taint checking

Taint checking is a Perl run time feature that tracks the flow of data inside a program. Perl marks as “tainted” data that are derived from user input or the outside world in general, such as command-line arguments, environment variables, or file or stream input. You won’t be able to pass any of this data to external programs because Perl will stop your program when you try to.

You can enable taint checking with the -T switch: