Thanks.
I am not very experienced in perl. I understand (I think) what you have said but can you give me a clue - pointer - as to how to check the input and avoid the problem?
Best regards
Take a look at Ovid's CGI Course. Pay special attention to the parts that explain using Taint mode and untainting data. Furthermore, searching Perlmonks for "Taint" produces a number of good links, as shown here.