We deduce from the fact that used @f rather than f in your fzero call that f is the name of a function rather than being the handle of an anonymous function. If that is correct, that f is a function, then in order to have a hope of working, f would have to consist of a call to evalin specifying 'caller' as the workspace, and the function so invoked would have to specifically evalin() 'caller' or assignin() 'caller' the variables it wanted to use. The caller of f would be fzero() and the caller of fzero would be the workspace that had the fzero() call -- maybe, but it could be that fzero calls one or more internal routines that would have to be worked around...
If you do not need to change variables in your current workspace, and only need to read them, then you would be better off using an anonymous function that passed the needed variables to a version of f re-written to accept the variables as arguments:
fzero(@(x) f(x,p,q,r), [1 100])
If you need to be able to change the variables in your current workspace, not just read them, then you are probably using poor coding practices.
