use feature 'say';
use List::Util qw( product reduce );

sub f4 {
    my $n = shift;
    return reduce {
        my $x = int( $n / abs($b) );
        $a + $b * $x * ($x + 1) / 2;
    } 0, @_, -product @_;
}

say f4( 10,   3, 5 );
say f4( 100,  3, 5 );
say f4( 1000, 3, 5 );