use Data::Dumper;
my @numbers = (15,5,7,3,9,1,20,13,9,8,15,16,2,6,12,90);
my @a = join ("," ,(sort {$a <=> $b} @numbers));
print Dumper(\@a);

__END__

$VAR1 = [ '1,2,3,5,6,7,8,9,9,12,13,15,15,16,20,90' ];