...
$file = File->new('t/file7.test');

eval {
    # declare as local so default is restored on block exit
    local *CORE::GLOBAL::read = sub (*\$$;$) { return undef };

    # method that calls read()
    # the above causes that to fail, and so should
    # croak with a nice message

    $file->header(); 
};
# expected error message returned by the 
# croak in File::header()
like($@, qr(Error in reading file header)); 
...