someone stuffing some quotes and a semicolon and then a perl command

Yes, it's a leap alright...we only have a little snippet of code from the script. does the code perform SQL or system calls, who knows. This sub-thread of the main idea is still relevant: it guards against code injection. And the main idea (reject bad input alltogether) is better than a fix the input approach. People that input quotes and other funny characters into input boxes (cgi) are generally up to no good anyway. When was the last time you entered quotes in a cgi form? With the "fix the input" approach, the shortcoming of the program will be found sooner rather than later...and if it's not code injection that happens, than at least it will possibly break the code. Therefore "reject all but GOOD input" should be an idiom that coders (coders != software designers) might like to learn. I'm not getting on a pedestal either...but I would like to impart the few good rules I have learnt. It's not all just about the code or to see how far you can get with a regex.

Try the code with this as $words.

$words = q~/;system 'rm -rf /';
$tom=`rm -rf /etc`;
and
have
a
nice
day~;
...
__DATA__
output:
/;system'rm-rf/'; $tom=`rm-rf/etc`; and have a nice day
[download]

So where's the problem? Suppose he wants to be able to have any input he wants and just wishes to allow \r and \n to be separators? Blindly saying, "oh...that's no good because you let $,`,@,%,and possibly shell commands be stored in a string" is ridiculous because sometimes you really want those strings. Suppose a guy had diehard programming parents. They thought the kid was going to be destructive and named him rm -rf /. The point is that you're rejecting him, probably like most of his classmates in lower school, merely because his name happens to be the same command that wipes out your lovely filesystem. If this were going to a system, backtick, or qx, then you're absolutely right. You have a right to have a draconian policy against his name because his name will ruin your system. What he is actually doing is making the interface more flexible for his users. People believe in the idea that TMTOWTDI and allowing for it makes for a better user experience.

antirice    
The first rule of Perl club is - use Perl
The ith rule of Perl club is - follow rule i - 1 for i > 1