sub compressPairsWithCount(&@) {
  my $cr=shift;
  my @aResult;
  my $x;
  my @aRuns=(0);
  my $i=defined($_[0])?-1:0;
  my $j=0;

  local $a=undef;
  local $b=undef;
  foreach (map {
    if ((defined($x) && defined($_) && ($x eq $_))
        || (!defined($x) && !defined($_))) {
      $aRuns[$i]++;
      ()
    } else {
      $aRuns[++$i]=1;
      ($x = $_);
    }
  } @_) {

    $a=$b;
    $b=$_;
    $_=$aRuns[$j++];
    push @aResult, $cr->();
  }
  return @aResult;
}

@aCompressed = compressPairsWithCount
  { '(' . (defined($a)?$a:'<undef>') . ','
      . (defined($b)?$b:'<undef>') . "=$_)" } @aData;
print "compressPairsWithCount: @aCompressed\n";

#outputs
compressPairsWithCount: (<undef>,a=4) (a,b=1) (b,c=2) (c,a=2) (a,d=1) (d,e=4) (e,<undef>=2) (<undef>,f=1) (f,g=2)