#!/usr/bin/perl
use strict;
sub foo($);   #declare foo as a sub that accepts 
              #a single scalar, to be defined later

foo;          #compile time error
foo 1;        #okay - no error

sub foo($) {  #actual foo implementation
  print shift;
}

##</code><code>##

sub foo($$);      #two scalar arguments
foo $bar;         #error
foo @bar;         #error
foo $bar, $baz;   #ok
foo $bar, @baz;   #no error, but does it do what you want?