my $data;
for($x=0; $x<$sth->rows; $x++) { 
  my $prev = $data;
  $data = $sth->fetch_hashref; 
  my $this = $data->{id}; 
}

##</code><code>##

my $prev;
while (defined my $data = $sth->fetchrow_hashref) {
  my $this = $data->{id};

  ...

  $prev = $data;
};