for(  
    'sue,fred,x(mary,jane)', 
    'x(a,b),c(d,e,f),g,h,i(j,k)', 
    'a,b,cdef(g),h,i(j)' 
) {
    print join' | ', m/( [^,(]+ \( [^)]+ \) | [^,]+ )(?:$|,)/gx;
};;

sue | fred | x(mary,jane)
x(a,b) | c(d,e,f) | g | h | i(j,k)
a | b | cdef(g) | h | i(j)