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

  local $a=undef;
  local $b=undef;
  foreach (map {
    defined($x) ? ($x eq$_?():($x=$_)) : defined($_)?($x=$_):()
  } @_) {
    $a=$b;
    $b=$_;
    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 = compressPairs { '(' . (defined($a)?$a:'<undef>')
                            . ',' . (defined($b)?$b:'<undef>') . ')'
                        } @aData;

print "compressPairs: @aCompressed\n";

#output
compressPairs: (<undef>,a) (a,b) (b,c) (c,a) (a,d) (d,e) (e,<undef>) (<undef>,f) (f,g)