There are many ways of calling code that is not immediately visible:

• If your package inherits from another package, Perl will look for methods in that other package too. Inheritance from a package foo is commonly done via use base 'foo'; or via direct manipulation of @ISA.
• Methods can be created dynamically. Class::Accessor does that, and you can do it yourself:

  eval q{sub foo{print "Hello World"}}
  
  # or, somewhat cleaner, with Perl checking your
  # syntax at compile time:
  {
    no strict 'refs';
    *{__PACKAGE__ . "::foo"} = sub {
      print "Hello World";
    };
  };
  
  # ... and many more methods
  [download]

• If there is an AUTOLOAD subroutine in the package, it will be called for every undefined subroutine:sub AUTLOAD { print "You called '$AUTOLOAD(@_)' via AUTOLOAD but it doesn't exist\n" };
• AUTOLOAD can also be inherited. This is a dangerous practice and I think Perl warns you if an inherited AUTOLOAD handler is called.

I think this catches all common cases of creating methods, but I might have overlooked something.