Perl's Taint Mode
Perl is a great language for web programming and one of the features that makes it well suited is Taint Mode. Perl’s Taint Mode, quite simply, restricts what the programmer can do with variables that are acquired from user input. If a programs relies on the user to specify something, it’s a bad assumption to treat those values as safe for use in operations that may affect the outside world, such as opening files and using tainted values in system() calls. Perl wont restrict you from using tainted values in innocuous things such as print statements but it does pass taintedness on to other variables that a tainted value touches. Furthermore, modules such as DBI support recognizing taintedness of variable for safety.
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