my @a = (1,2,4,8,4,5,6,7,8); # sample data
my ($current, $previous);
my $tmp_max = 1;
my $max = 1;

while (@a) {
  $previous = shift(@a) ;
  $current = $a[0] or last;
  if ($previous + 1 == $current) { # or $previous < $current if numbers may increase by more than 1
    $tmp_max++;
  } else {
    $tmp_max = 1;
  }
  $max = $tmp_max if ($tmp_max > $max);

}
print "$max\n";