use Carp;

sub sum {
    croak 'Usage: sum($first, $second)' unless @_ == 2;
    my ($first, $second) = @_;
    return $first + $second;
}

sub call {
    my ($sub, @args) = @_;
    croak 'Usage: call(\&sub, @args)' unless ref $sub and ref $sub eq 'CODE';
    return $sub->(@args);
}


$\ = "\n";

print call(\&sum, 1, 3);

# Error happens in call() [line 12], because that's where sum() is called!
print call(\&sum, 5);