use strict;
use warnings;

sub compress {
  my $x;
  map {
    if (!defined($x)) {
      defined($_) ? ($x = $_) : ()
    } else {
      defined($_) && ($x eq $_) ? () : ($x = $_)
    }
  } @_;
}

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 @aData;
print "compressed: @aCompressed\n";

# outputs
a b c a d e undef f g