I understand that the prototype sub f ($) {} makes sure f is called with exactly one argument.

Sorry, but that's not true:

#!/usr/bin/perl
use strict;
use warnings;

sub f($)
{
        print 'f called with ',0+@_,' arguments: ',join(', ',@_),"\n";
}

sub g
{
        print 'via g: ';
        &f;
}

f(42);
&f(42);
&f();
&f(42,99,333);
g(42);
g();
g(42,99,333);
[download]

Output:

f called with 1 arguments: 42
f called with 1 arguments: 42
f called with 0 arguments:
f called with 3 arguments: 42, 99, 333
via g: f called with 1 arguments: 42
via g: f called with 0 arguments:
via g: f called with 3 arguments: 42, 99, 333
[download]

See perlsub for what happens here, and why perl does not complain.

Alexander