Using the one() junction

my $threshold = 70;

sub authorize_movie(@grade) {
  if (one(@grade) < $threshold) {
    say 'you may go to the movies';
  } else {
    say "sorry you had more than one grade below $threshold";
  }
}

sub load_grades {
  qw(50 80 90);
}

my @grade = load_grades();
authorize_movie(@grade);

To test for 2, 3 or othe amounts of grades below threshold, you need List::Util::filter() - a standard part of Perl 5.