• Actually you can redefine subroutine by using eval "sub func {'...'}"

That would create a new function at runtime, but it wouldn't follow the rules of dynamic scoping (see MOPT-03 for details).

As you've used it, the eval() just binds a new function to the name 'func'. To be dynamic, the new version of func() would have to be restricted to the evaluation context of the function that defined it:

    $VAR = "old VAR";
    sub func {return ("old func()")}
    sub show {printf "VAR='%s', func()='%s'\n", $VAR, func()}
    
    sub redefine {
        eval 'sub func {return ("new func()")}';
        local ($VAR) = "new VAR";
        show();
    }
    
    show();
    redefine();
    show();
[download]

returns:

    VAR='old VAR', func()='old func()'
    VAR='new VAR', func()='new func()'
    VAR='old VAR', func()='new func()'
[download]

which shows the difference between redefining a globally-scoped function and re-binding a dynamically-scoped variable.

• and better yet is just to use anonymous subroutines and use ordinary variables to switch between different ones:

Yes, that would work:

 
    $VAR = "old VAR";
    $FUNC = sub {return ("old func()")};
    sub show {printf "VAR='%s', func()='%s'\n", $VAR, &$FUNC()}
    
    sub redefine {
        local ($FUNC) = sub {return ("new func()")};
        local ($VAR)  = "new VAR";
        show();
    }
    
    show();
    redefine();
    show();
[download]

returns:

    VAR='old VAR', func()='old func()'
    VAR='new VAR', func()='new func()'
    VAR='old VAR', func()='old func()'
[download]

which does the right thing. But to get the effect of a dynamically-scoped function, you have to use a dynamically scoped (aka: local) variable, and I did say that in the original article.

HOWEVER.. check out the next reply to see how I was, in fact, wrong.