Simple CAPTCHA Modification Needed for formmail.pl Script to Prevent Spam

I need a simple modification to my [login to view URL] scripts. I have several websites using formmail.pl. They are being spammed. I need someone to write a script preferably in php that will not let the form be submitted unless they pass a CAPTCHA type 5 digits visual code. CAPTCHA images must be attractive looking and be appropriate for business sites. If there are multiple files involved, I need the script writer to load a directory with the code, images etc and modify 2 forms on one site. I’ll then make the modifications to the other sites. It would be ideal to view a similar working script the bidder has developed before the project is authorized, but not required. 100% payment will be made upon project completion and a successful 10 day testing period.