my $first_row = 500;
my $window_size = 50;
my $location = 1;
while (my ( $col1, $col2) = $sth->fetchrow_array() ) {
    next if $location < $first_row;
    last if $location >= $first_row + $window_size;

    # Do whatever I need to do with the data
} continue {
    $location++;
}