use strict;
use warnings;

package override;

sub import {
   my ($class, $sub_name, $sub) = @_;
   my $pkg_name = caller();

   my $pkg_sym = _pkg_symtab($pkg_name);
   delete $pkg_sym->{$sub_name};

   if ($sub) {
      no strict 'refs';
      *{"${pkg_name}::$sub_name"} = $sub;
   }
}

sub _pkg_symtab {
   my ($pkg) = @_;
   my $p = \%::;
   $p = $p->{"${_}::"} for split /::/, $pkg;
   return $p;
}

1;

##</code><code>##

use strict;
use warnings;

open(my $fh, '<', $0);  # Calls CORE::open

use override open => sub { print "Hi\n"; };
open(my $fh2, '<', $0); # Prints Hi

use override open => sub { print "Ho\n"; };
open(my $fh4, '<', $0);  # Prints Ho

use override open => undef;
open(my $fh5, '<', $0);  # Calls CORE::open

##</code><code>##

>perl test.pl
Hi
Ho