Here's an example. You assign a reference to your own subroutine to the CORE::GLOBAL namespace for the function in question within a BEGIN block. In this case, the custom open() will only accept the 3-arg form of open.

use warnings;
use strict;

BEGIN {
    *CORE::GLOBAL::open = sub (*;$@) {
        print "in my open...\n";

        # the following line calls the built-in open() function
        # directly with the params your sub received

        CORE::open($_[0], $_[1], $_[2]);
    }
};

open my $fh, '<', 'blah' or die $!;

print <$fh>;
[download]

Output:

in my open...
file contents
[download]