-FUD is intended (by the authors of Perl) to emulate the  -whatever syntax of Tcl/Tk options. The leading '-' is appended to the string into which the  FUD bareword is converted in the presence of the fat comma, any stricture against such conversion being locally nullified.

+FUD is intended (by various monks replying) to illustrate the effect of the unary  + operator on the string into which the  FUD bareword is converted in the presence of fat comma: none at all. Contrast this with the effect of the  ! logical negation operator in the  !FUD case.

To answer your question more generally, the syntax +FUD and -FUD (and !FUD and ~FUD and maybe some others) means that someone had a bright (and well intended) idea and far too much time on their hands, and now we're stuck with it.