sub compress(&@) {
  my $cr=shift;
  my$x;
  my @aResult;

  local $_=undef;
  foreach (map {
    defined($x)
      ? defined($_) && ($x eq $_) ? () : ($x = $_)
        : defined($_) ? ($x = $_) : ()
      } @_) {
    push @aResult, $cr->();
  }
  return @aResult;
}

my @aData=(qw(a a a a b c c a a d e e e e), undef, undef, qw(f g g));
my @aCompressed
  = compress {'*' . (defined($_)?$_:'<undef>').'*'} @aData;
print "compress: @aCompressed\n";

# output:
compress: *a* *b* *c* *a* *d* *e* *<undef>* *f* *g*