#!/usr/local/bin/perl -w
use strict;
use subs 'open';
no strict 'refs';
sub open (*;$@) {
  my $file=shift;
  my $args=shift;
  print "My_open\n";
  # Do whatever you want
  CORE::open (*{$file}, $args);
}
open OUTPUT, ">test2.txt";
print OUTPUT "Hi there.\n";
close OUTPUT;
__END__
$ perl override.pl
My_open
$ cat test2.txt
Hi there.