sub combine_factors { my $list = shift; my $factors = {}; for (@$list) { for (factors($_)) { ++$factors->{$_}; } } $factors; } my $fa = combine_factors(\@a); my $fb = combine_factors(\@b); ($fa->{$_} ||= 0) -= $fb->{$_} for keys %$fb; my @c = map +($_) x $fa->{$_}, grep $fa->{$_} > 0, keys %$fa; my @d = map +($_) x -$fa->{$_}, grep $fa->{$_} < 0, keys %$fa;