sub fudge_method {
  my($obj, $method, $sub) = @_;

  ## you'll probably want to extend this for other data types
  $_[0] = bless {%$obj}, init_class(ref $obj, $method, $sub);
}

{
  my $cnt = 0;
  sub init_class {
    my($class,$method,$sub) = @_;
    my $name = $class."::_tmp".$cnt++;

    no strict 'refs';
    @{$name."::ISA"}     = $class;
    *{$name."::$method"} = $sub;

    return $name;
  }
}

{
  package foo;

  sub new { bless {@_[1 .. $#_]}, shift }

  sub dostuff { print "I'm doing stuff [", %{$_[0]}, "]\n" }
}

my $o = foo->new(field => 'var');

print "pre fudge  - ";
$o->dostuff();

fudge_method($o, 'dostuff', sub {
  print "I've done stuff [", %{$_[0]}, "]\n";
});

print "post fudge - ";

$o->dostuff();

my $o2 = foo->new(this => 'that');

print "new object - ";
$o2->dostuff();

__output__

pre fudge  - I'm doing stuff [fieldvar]
post fudge - I've done stuff [fieldvar]
new object - I'm doing stuff [thisthat]