my @union = array_union_sort(\@first, \@second);

sub array_union_sort {
    my $aa = shift;
    my $bb = shift;
    my %union;
    my $e;
    foreach $e (@$aa) { $union{$e} = 1; }
    foreach $e (@$bb) { $union{$e} = 1; }
    my @union = keys %union;
    @union = sort { $a <=> $b } @union;
    return @union;
}