Here's my shot at it, assuming "turkey" isn't always going to be the same pattern/word.

while(<DATA>) {
    s#(elephant\s+\*\s+)(.*?)\w+$#$1$2fox# if (!/-ugly/);
    print;
}

__DATA__
elephant * -ugly /leave/this/alone
elephant * /only/change/->/turkey
elephant * turkey

elephant * -ugly /leave/this/alone
elephant * /only/change/->/turkey

elephant * -ugly /leave/this/alone
elephant * turkey
[download]

This is somewhat of a kludge, as I wanted to use a negative look-behind on the "-ugly", but I couldn't get it to work.

If "turkey" is always going to be "turkey", then the solution is a lot easier: s/turkey$/fox/