Ever wanted to evaluate a user-provided expression, maybe from a form input or otherwise, and return the result? eval($expr) is dangerous because eval(open $out, "> file.what"); will open the file, infact, eval(everything) will pretty much do everything. checking though everything is tedious, and manually parsing everything seems like a waste of time.
Well. I found the Safe module and spent a while figuring it out, Safe is included in all present distributions of Perl. The main thing to figure out is which opcodes to allow for a reasonable eval and nothing dangerous. Well, here they are to save you time: