Just to document an adaptation on Cody's approach, that I´ve just tested with success:

Instead of

sub half {
  my $number; 
  if (ref($_[0]) eq 'MyPackage') {
      my $self   = shift();
      $number = shift();
   } else {
       $number = shift();
   }
   return $number / 2;
}
[download]

We can make the adaptation of all perl4-ish functions possible just by adding a single line, on the top of each function's code:

sub half {

  if (ref($_[0]) =~ /MyPackage/ ) { my $self = shift; } 

# as the former Perl4-like functions don´t need the $self object, this
+ line's role is to clean up the first element of the @_ array! And, s
+o, make things ready for the shifts your code already is set to!
  
  my $number = shift;
  
  return $number / 2;
  
}
[download]

Detail: I call all my methods libraries MyPackage, Mypackage2 etc, so by testing with this regexp, I don't even have to worry to adapt this one-line according to the library I´m converting into module! Just adding it all over the subs, and finally enter the OO world!