Using a coderef at all bypasses prototypes. (As does calling a sub as a method.)

If you need to check the number of arguments passed to a sub, do it in the sub! Prototypes are just to help the Perl parser parse argument lists more sanely.

sub execute {
   @_==3 or croak "execute() needs three arguments!!";
   ...;
}
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As others wrote already there is no way to make this work. The important thing to remember is that prototypes are NOT subroutine signatures! Prototypes do not exist to allow you to specify the number and type of arguments! They exist to allow you to create (very occasionally) a subroutine that works as a builtin. Take push() for example. If you pass (@arr, 1,2,3) to an ordinary subroutine, then the subroutine receives a list containing all elements in @arr followed by 1, 2 and 3 and has no way to modify the @arr array. push() on the other hand receives the array and then the 1, 2 and 3 and can modify the array.

If for whatever reason you feel the need to create a subroutine that is used the same as push(), you can use a prototype to tell Perl to change the way its parameters are handled, but you should not attempt to use it to validate that a subroutine gets the expected number of parameters!

The only prototype an ordinary programmer should ever use is the empty one for constant subroutines (and even then  use constant ... is better) and MAYBE ($) for subroutines that are to be used in mathematical expressions. All other prototypes are best left to a few module authors.

The (&) prototype is useful if you're writing subs that expect to be passed coderefs/callbacks. And (_) can be useful too.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

\&execute and \execute do two different things.    \&execute passes a code reference to the subroutine threads->create while \execute executes the subroutine execute and passes a reference to the return value.

Hi ISAI,

On a meta level (and regarding your last posts :)

I think Damian's Perl Best Practices could be a valuable read for you!

It helped me a lot to get a much more profound orientation in Perl.

Cheers Rolf