Simply remember your "previous" element before you fetch the next element.

my $data;
for($x=0; $x<$sth->rows; $x++) { 
  my $prev = $data;
  $data = $sth->fetch_hashref; 
  my $this = $data->{id}; 
}
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Maybe you should use a different loop style than the C loop for iterating over your results to avoid the polling of $sth->rows:

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

  ...

  $prev = $data;
};
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