package orig::functions;
BEGIN {
  use Exporter ();
  use vars qw(@ISA @EXPORT_OK);
  @ISA         = qw(Exporter);
  @EXPORT_OK   = qw(func1 func2 func3);
}

sub func1 {
  print "hello";
}
sub func2 {
  print "kind";
}
sub func3 {
  print "world";
}

##</code><code>##

package replace::functions;
BEGIN {
  # import the original functions, but not the ones
  # this package is going to replace
  use orig::functions qw(!/func1|func2/);
  use Exporter ();
  use vars qw(@ISA @EXPORT_OK);
  @ISA         = qw(Exporter);
  # let the original functions be exported as well
  # as the new ones
  @EXPORT_OK   = (@orig::functions::EXPORT_OK,
      qw(funcx funcy)
    );
}

# func1 is going to be replaced with our own
sub func1 {
  print "goodbye";
}

# func2 is going to be replaced with our own
sub func2 {
  orig::functions::func2();
  print "(haha)";
}

# func3 is reused from orig::functions unchanged

# a couple new functions
sub funcx {
  print "new x";
}

sub funcy {
  print "new y";
}

##</code><code>##

# program A

use orig::functions qw(func1 func2 func3);

print func1(), func2(), func3(), "\n";

##</code><code>##

# program B

use replace::functions qw(func1 func2 func3 funcx);

print func1(), func2(), func3(), "\n";

print funcx(), "\n";