Depending on what exactly you want to do with undefined values, this works.

#!/usr/bin/perl -w
@a=qw(ala ma kota tytus ma kolty a pies ma ale);
print join ", ", uniq( sort @a),"\n";

sub uniq {
  my $last;
  map { (defined($last) && $last eq $_) ? () : ($last = $_) } @_;
}
[download]