what is the phone number to report a fraudulent app purchase

If the purchase is on your iTunes account you can contact iTunes Support via http://reportaproblem.apple.com or http://www.apple.com/support/itunes/ww or https://www.apple.com/emea/support/itunes/contact.html

If it's not on your iTunes account but shows on your credit card statement (i.e. it isn't just a phishing email that you've received) contact the card issuer.

World wide support phone numbers can be found here:

Contact Apple for support and service - Apple Support

There is no specific number for fraudulent purchases.

However, generally to report a fraudulent purchase you go through the http://reportaproblem.apple.com website to first verify the purchase is actually real and not just a scam or phishing email. You can report a problem with the purchase directly from that website if it is real.

Read here for more information dealing with unknown purchases:

If you see 'itunes.com/bill' or an unfamiliar charge on your bank, credit card, or debit statement - Apple Support

and

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

What makes you believe that there has been a fraudulent app purchase? If you received an email message that appears to be a receipt for an app that you did not purchase, do not click on any of the links in the mail. It is most probably a phishing attempt. See this to help identify fraudulent messages. Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

IF you do not see a purchase when you check your iTunes purchases in your iTunes account, or you do not see a charge on your bank account, then forward the message to Apple at reportphishing@apple.com and then delete the message.

Contact Applecare and request to speak to iTunes

https://www.apple.com/ca/contact/

Try login to reportaproblem.apple.com to check purchases or for requesting a refund

800-MY-Apple but they dont really have a fraud dept...