What's wrong with having multiple M-files? An option would be to move all of them into a subfolder called \private\, such that only the main fail in the parent folder can access them.
If you want to have them in one M-file, you need a so called wrapper:
function varargout = MyWrapper(Func, varargin)
varargout = cell(1, nargout);
switch Func
case 'SubFunction1'
varargout = SubFunc1(varargin{:});
...
otherwise
error('Unknown function: %s', Func)
end
function [a,b] = SubFunc1(c, d)
...
Another method is to offer a set of function handles:
function FcnList = MyWrapper
FcnList.Function1 = @Function1;
FcnList.Function2 = @Function2;
...
function [a,b] = Function1(c, d)
...
function [q,p] = Function2(z)
...
Now you obtainthe handles in the first step inside the main function, and call the function handles afterwards:
FcnList = MyWrapper;
[a,b] = FcnList.Function1(c, d);
...
