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>;

##</code><code>##

in my open...
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