FormMail - value1=value2

Hi, I hope a really simple question with an equally simple answer...!

Trying to get FormMail to recognise that "M_recipient" means "recipient", for the purposes of WorldPay which can only pass on fields that begin M_. Have tried M_recipient=recipient (wishful thinking).

Apologies if I'm posting in the wrong ng, FormMail is Perl based and I live in hope..

Thanks in advance, Adrian.